I (25M) wasn't honest about a detail when discussing something that happened during a no contact phase before we got together. How do I move forward with my girlfriend (23F)?

So Me (25M) and my girlfriend (23F) first met through mutual friends in university about 3 years ago. During that spring semester we had kind of an ongoing fling or one might even call it situationship and towards the end of the semester there was definitely some chemistry. But I was still uncertain and we were far from actually getting together. I basically handled this uncertainty very immature and decided to basically not text her at all during the 3 Month long summer break. This was very different from her side because she still says how much she kept thinking about me and that she had hoped all the time that i would text her. In this summer break i went on a boys trip with my friends and on that trip I had a very shallow hook up that some people might consider gross. It was my first and only time doing something like this and I am not proud of myself for this.

When university started again I realized how stupid I was in not giving it a shot and luckily me and my girlfriend then got together. Since then I fell in love with her deeper and deeper every day for the last 2.5 years. Still I handled the situation wrong and only told her about the holiday thing about 4 Months later, when she asked if something had happend during our no contact phase for the first time. That was how i decided to handle it back then, I told myself if she asks then you tell her. I know that I should have told her the moment I knew I wanted a relationship. Surprisingly it wasn't even that big of a deal for her back then, I guess this was somewhat because of the crazy honeymoon phase. This was almost 2 years ago.

So during the last 2 years it has become bigger and bigger of a deal for her. More things triggering her to think about it showed up. I always tried to reassure her that it meant nothing but still this whole story was like this big mysterious thing and I saw her struggle with not knowing the details. I now know that I should have just been completely honest and tell the story way sooner. She only knew that I had slept with someone and that it was a one time thing but no further details. About 8 weeks ago she couldn't take it anymore and asked about the details. So she went asking one question after another and I answered truthfully. This went on until one particular answer just left her in shock. She wasn't able to look me in the eye and I was devastated. After a few minutes of her looking at me filled with disgust she asked one more, rather irrelevant question, basically if we did oral or not. And thats when, in the heat of the moment, I fell back to being to cowardly to just own my actions because I thought this would be too much. I had received oral back then but I told her i did not.

Now the last 8 weeks were rough. We were both pretty devastated since apart from this issue our relationship was great, we love each other and we were both thinking that we might go all the way. We used to talk a lot about our future, kids and also marriage. I tried my best to be supportive and build her back up, but since I caused all the harm I didn't really find a way. We were on the edge of breaking up a couple of times. Now it seems like she is finally coming to closure with the details. Still it turns out the biggest problem is not the thing itself but the fact that I was able to not tell her at all for 4 Months and that later during the 2 years that she knew about it I somehow created a wrong image in her head. Although until I told her, I never lied to her face about it, I said things that gave a wrong impression of me. I talked to her about not thinking much about shallow hook ups and over time this created this impression that I was not a guy to do something super shallow or even gross. I didn't do this on purpose but more in order to make it as manageable as possible to her and of course I wanted her to think a lot of me, which ultimately did so much more damage. In general I handled the whole situation in an insecure, cowardly and dishonest way. Again I had just decided to tell the details once asked. I can see how this was immature and dishonest and that this is a big part of the reason her reaction to the details was this big. Basically now all trust in me and my honesty is gone and she says she is not sure if she can trust me again. She said she would like to move on and continue the relationship but it is kind of a gamble to her because she cannot know if from now on I am honest with things no matter how uncomfortable they are and that maybe she might only find out in a few years once something comes up that allows me to prove that i have learned from my mistakes.

I told her that I know this side of me that trys to regulate things once they get uncomfortable, I know how much harm it did and that I want to be better and more mature. I want to learn from my mistakes. After that I actually remembered this one lie about the oral that I told and I feel like this would be the first opportunity to be better in telling her. On the other hand I am not sure because i don't even know if she remembers asking me this and I also feel like it might do more harm than good since it feels like she is finally getting over the details, so adding more might be counter productive.

I am really not sure what to do, I love this girl and her kind and intelligent soul. I hate that I was such an ass before getting together and that I then continued acting like an immature coward during the relationship. I want to be the person she deserves and need advice on what to do?

TL;DR: 8 Weeks ago, reveiling all the details of a story from my past that is a big issue in my current relationship caused my girlfriend to be in shock and the relationship is on the edge ever since. During that first few minutes of shock she she asked one more question about a small detail that i didn't answer truthfully. She seems to slowly be getting over the details but her biggest concern right now is me not being honest when it gets uncomfortable and not knowing if i can change. I don't know if I should tell her about this not truthfully answered question from 8 weeks ago.