Help! My GF thinks I cheated on her.

Hello everybody, I am desperate:
I have a wonderful 3 year relationship with this amazing girl. I love her deeply, and I've been starting to think about starting a family with her and everything. I am a 31 year old male.

I used to be an a**hole in previous relationships, cheating on my two ex-GF several times. I also told my current GF every single detail about my cheating in previous relationships, so that she sees the whole picture. Hence: she knows my ways and how I used to be.

Honestly people, I do not have to lie in an online forum with a nickname, I know is hard to believe that a cheating man can change his ways, but in my case, it is truth.

Everything was perfect, until yesterday.
I forgot my iphone at her place, and asked her to return it to me in the morning. She checked my Whatsapp messages and stumbled upon on particular chat I have with a girl I used to date 4 years ago. She lives in Spain (I live in South America), and we've been sending us kinky texts, fooling around, flirting, etc. I even told her that I was going to Germany for a PhD I am planning to do this year, telling her that I would love to meet her (which is not true because, in the first place, I do not want to meet her!, and, secondly, she has a boyfriend). She sent me some nudes too, so, during those 4 years we've been in touch, chatting every couple months, flirting and texting. I never seen her this entire time, and she has a boyfriend in Spain and lives with him. All in all, it doesn't mean anything neither to me not to her, but the problem is that my GF read those chats, and now she is 100% sure not only that I cheated her with this spanish girl, but also that I am cheating her in the real world on a regular basis!

So, there is the issue that she broke my intimacy by sneaking into my Iphone (I do not have a password, these chats were so meaningless to me that I didn't even bother to erase them; they are 4 or 5 months old). Ah, I am very anti tech so, my Whastapp is composed by 100+ chats that i never erased (not that you think that somehow these chats with this girl where "special" or something like that).
So, in the end. She broke with me. She doesn´t want to hear from me. And I did not cheat her (physically)!!! I understand her altough, because I would hate if I catch my GF flirting via Whatsapp with some jerk. I know I made a mistake, but I don´t want to lose her.
I would appreciate your advice, I am desperate and want to fix this. I really love her and want her back.

Help! and Thanks!