Trust rebuild - My girl is afraid she can never trust me again.

Hi guys.
My story is very long one, so I will try to make it as short as possible, but the complexity demands that I include some crucial details for you guts to get the best picture possible.
I could use all the advise you guys can give me.

My now ex-girlfriend and I had been together since 2012 when we got out of highschool. Neither of us have ever had anyone else, and our love was so over the top for many years, and everybody thought we would stay together forever. When we began college, something in me changed, I had trouble figuring out what, but I think that I was afraid what the next step for us would be, when would we be ready for marriage or children - cause i really wanted that. But I doubted, and I started doubting that she loved me, and would stick we me when I was so confused about us. I felt guilty because I was mentally absent, and wasn't the boyfriend she deserved, cause she is truly amazing. So I took a decision of moving out of our apartment, because I wanted to have some time to figure out what happened inside of me, and why I was doubting this. Because I wanted to be with her.
She immediately interpreted as if I was breaking up with her, and I never said those words. I never meant those words. I just couldn't stay where I was at, something had to change so I could se clearly what needed to be done in our life. Cause we needed to do something to deal properly with our problems. This was in late december (just at christmas - i know... worst timing)
My girlfriend, lets call her (P), she tried do anything she could to save us, because she couldn't understand how I so suddenly and out of nothing just came to this decision. She totally lost everything in that moment, and she did all that was in her power, she suggested couples therapy, that I moved and we tried to stay as a couple just not living together, she wanted to try all kind of things. And I could not engage in all that, I couldn't see what I needed, to realize it my self.

But I finally moved out in March 18, and this was where I felt like I could start figuring out my self. And it was a very long proces, where i eventually had to go see a psychotherapist, to find out what my real problems were. Of course what I found out in that proces is very complex and not easily explained. But it turned out, that I had a deep anxiety of not being good enough for her, and that she would eventually find out about this and leave me, and because of that I tried to control everything that happened in our relationship, so she could only think the things that I wanted her to, and never leave me. I could not tell her my deepest thoughts and feelings, because I was afraid she would no longer love me, and that started all of the problems that we had. Which I thought was kind of her fault at first. But suddenly I could see the pattern. And I knew what needed to change for us to get back together, and live the rest of our life together. Cause she really indeed is the girl for me, no other woman can tempt me. I want to grow old with her. And she felt the same way. Until one point....

Her story in all of this is, that she stayed in our apartment and had to live there alone. For a whole year, she was trying to convince me to come back to her, and to start doing something with a therapist to get to the bottom of it. And her feeling was that she did not know if I wanted to be with her or not. My biggest regret is that I was not able to make sure that she knew that I wanted her all along.
So, Oktober 18, she sent me a letter, where she wrote about what she went through for almost a year now, and how that has led to her not wanting to be with me anymore. She felt she deserved better, and that she now felt she could let go of me, she still loved me, but no longer wanted to be with me. And the worst things is, that the days before I had a breakthrough in therapy where I realized how we could start working together to understand my mental state, this whole year, all of our problems and how to move on from it. But then two days later I got the letter.

And from that point it went down the drain, because she couldn't wrap her head around that now when she moved on I was suddenly ready. So she needed some time to think about it, and she took one month, and then she said she needed another month, and I gave her all the time she needed, I knew what she had been through and I did not want to push her at all. And then when January 19 hit, she told me she still felt the same way as she wrote in the letter. Something had changed for her.

I could not understand it. Now the tables had turned, I was the one standing in the dark and not knowing what to do. I could not get how she could just let our love die that way. I thought it was different than what I did, because I needed to find out somethings that could now help us with our problems. She thought I was seeing other girls but I was not. And when I told her that a couple of days ago she was so surprised, because she was sure of it. And now I find out she is seeing another guy, and they have went on to everything together already. Where I could never even deal with the thought of being with another than her. And as much as it haunts me and I picture me the two of them together, I still just wanna be with her.

And where we are now is that all of the work I have been doing for my self this past year, about myself, how I think and feel, and how it relates to our relationships, she dont any of that. Her decision was made without knowing that. So I feel my last chance is to tell her, how I feel, and how I am sure about us now, on a whole other level. I want to take the next step with us, and I am ready to marrying her without a single moment of doubt. I want to clear her head of all worries she has, and explain what happened in that year where she was feeling so much pain. And I want to rebuild our relationship, now that I know all the places where I need to be extra careful and have special attention.

The problem is, when I tell her these things she doubts that it is so simple, that I can just be so different now. And she says that she loves me deeply still, but cant think of me as her boyfriend anymore, and even if we tried she just knows that she would never be able to trust me again. It is impossible to rebuild her trust to me to that point where we could be a couple again. She would always fear that at some point in the future I would do the same again, and leave her in that way again.

My problem, and my final question is how I can show and tell her, in a way where she truly feels it deep inside, that I would never do the same, and that our trust can be rebuild. Stone by stone, step by step, day by day I will lay my heart in proving to her that she can trust me. But how do I make sure that she will understand this and feel it herself, so it isn't just empty words. Cause they're not. I am ready to whatever it takes, I would go through hell for her. But I cant seem to crack how I convince her that she should take this last chance with me let our love flourish as I know it will. And I do believe that she feels the same, but she has come to a point where she is telling herself that it is to dangerous, because she can get hurt. And because of that she it neglecting that side of her that still wants to build a family with me. But I will not hurt her. And i have no clue anymore to what I could do to prove to her that I am willing to anything, and I will do everything in my power to make her trust and love me unconditionally again. I feel this cant be true, when our love is so strong when things are going well. We were perfect for each other, and love like that needs to be saved and nurtured. I cant imagine her and me not being together, and I am so afraid of what will become of me if she decides that it is over for good. I am afraid that at that point I can no longer stay on this earth knowing we are apart. And I fear that scenario so much......

Please help me... Time is running out, and the true love of my life is floating away because of my stupidities and unawareness of what went in my mind.