Long-term girlfriend but in love with my straight best friend

Hi, I was wondering if anyone could offer any advice on my problem as it's absolutely taking over my life at the moment.
I've just left university with a girlfriend of 2 years who I am very much in love with. Until recently, our relationship was incredible and I have always believed she is perfect for me and who I will end up married to.
The problems started when I realised late last year that I am bisexual, something which completely hit me out of the blue and that I did not know how to express to anyone. I finally found the courage to tell my best friend (male) and he was so supportive, saying that it was fine and he had questioned his sexuality before too like a lot of people. It became more of a problem when I started getting feelings for this best friend. We have always been extremely close. We tell each other everything and are very tactile - wrestling, cuddling, etc - and occasionally seemingly a bit flirty. My girlfriend often used to joke that I prefered spending time with him (as we all lived in the same house). My feelings for this best friend got really strong to the point where it started affecting my relationship with my girlfriend, as I would stop wanting to have sex with her as often and would get really anxious out of guilt. I could never tell her about the situation because, at the end of the day, I was still madly in love with her and did not want to ruin anything.
So a couple of months ago I decided I would make my feelings known to my best friend after a few drinks just to see if I was being crazy or whether he actually liked me too. He told me he wasn't interested in me in that way and did not want to experiment at all but still wanted to be my best friend. Honestly nothing has changed at all between us since my revelation and we are still as close and tactile as we ever were. I have brought up my feelings for him on two occasions, the second time going into deeper discussion about everything, so he is definitely aware. My girlfriend however is not at all.
Since we have all left university, I have found my feelings for this best friend grow to an unbearable level and my feelings for my girlfriend have somehow been diminished by this. I now no longer miss her as much because I am so busy thinking about him. I now think about him sexually in a way that I don't about her anymore. Given all of this, I have some questions...
1. What has happened? Have I literally done a 360 from straight to gay? I have never had feelings for any other man apart from my best friend but have fancied many girls. I am also still very much emotionally in love with my girlfriend, just not as sexually attracted anymore.
2. How do I get over my best friend? He has made it clear that he's not interested so why can't I accept that and move on?
3. Do I tell my girlfriend and if so how? Considering he's not interested and I might not fall for anyone else, is it necessary to hurt her?
Plus if I tell her I'm bi, I think she will assume straight away that I have feelings for my best friend, which if we were somehow able to get our relationship on track, would make it basically impossible for me to see my best friend without a huge amount of jealousy involved.4. Will my best friend ever just go back to being my non-romantic best friend in my mind? And will my girlfriend ever go back to what she was in my mind before?

Thank you so much to anyone who bothers to read even half of that. I know my questions are a bit ridiculous considering no one can read my mind or the future, but I was wondering if anyone has any advice based on their own personal experience or interpretations of the situation.

Note: I've basically established in my own head that I must be bi and need to experiment with a man at some point soon just to get this sorted in my own head as I have never done anything with a man before. I'm just absolutely terrified that telling my girlfriend this will end our relationship.