I (21M) have lied to my girlfriend (21F), broken her trust and made her think I cheated on her. What can I do to help rebuild her faith in me?

We’ve been together for 4 years. We met in secondary school, got together in sixth form and live together in University. We’ve just rented a two bed flat together for next year.

I went to London from our home town to see a research tour. On the train I met someone from my primary school friend group. She (21F) and I haven’t seen each other for 10 years. We sat opposite each other on a train table and caught up for the 5hr journey.

She said we should get a drink whilst I was in London and also offered to let me stay at her flat on a couch. I declined as I was staying in a YHA. She gave me her number and told me to text her to arrange something.

During the conversation she mentioned how she’d recently split up and I mentioned my girlfriend.

In the evenings after getting to the hostel I rang my girlfriend and spoke to her about meeting this friend and going out for drinks with her. She mentioned she was uncomfortable with that and I mentioned I had no interest in her.

As an aside, I find it hard to make friends and there are only a few people (My girlfriend and a friend from primary school who was also friends with this girl before) I’ve met who I found it easier to talk to. Like I didn’t need to think as much. Nowadays I find it pretty lonely and I feel a bit sad and like a loser that I cant really talk to people.

So although my girlfriend was uncomfortable I kind of wanted to go out for these drinks (with no ill intention) because I wanted to catchup more and hopefully we could be friends in the future.

Another aside, I have OCD and ADHD. These aren’t excuses but I think give an insight to the way that I felt I had to speak and stuff.

After I went for the tour the next day, I messaged my girlfriend about wanting to meet up with this friend. However, I felt nervous and compulsion to lie to make it seem ‘completely’ like i had no interest. The lie i came up with was about work connections via her dad. It was very obvious. 

I asked her several times “what do you think?” Until she basically gave in and I messaged the girl and agreed a time.

Then whilst we were out for drinks, I wanted to message her honestly at each step about what we were doing but I felt a compulsion to seek reassurance that she wasn’t annoyed at me going out for drinks one on one with this girl.

Obviously, she was upset and not annoyed so my seeking reassurance (asking her how her dinner was and then asking her to check a date) seemed very suspicious and upset her more. She then messaged me asking exactly what I was doing and where and asked why I wasn’t answering quickly.

At the time, I was searching for a toilet and talking to a doorman about a toilet. I then told her I was just searching for a toilet. Then I found one and walked back to my hostel.

She hugged me goodbye, which I didn’t refuse and then I messaged my girlfriend. We didn’t really speak about it over text because I could tell she was upset. Rather than addressing it I talked about other things. She then later asked to call.

Then we spoke and I told her about the hug and she told me she felt sick the entire time and still does now.

Today, I got back and we met in person to speak. She asked me about my lie, which I admitted and explained that I felt very anxious and worried and so lied. This of course just made it worse.

There’s lots I’ve missed but I know its entirely my fault and I behaved like an asshole. My mental conditions aren’t justifications and I wish I was just open and honest rather than avoidant.

She told me every time she thinks of me and the girl she feels very sick. She has said if she can’t get over this she will want to break up.

I feel terrible that I lied, what can I do to regain her trust?

Thanks

tldr: Lied to my girlfriend due to anxiety / OCD (not an excuse) in an extremely suspicipus way and want to repair relationship.