Hey guys, just wondering if anyone had any advice as to how to cope with relationship anxiety.
Bit of a background story, my girlfriend was with her ex for about a year and a half. Towards the back end of their relationship, he wasn't treating her the best in terms of what she wants from a relationship, so started subconsciously 'window shopping'. At the same time, I, who was last in a 3.5 year relationship about a year and a half ago, started getting feelings for her. We have been best friends for about a year before getting feelings for each other and acknowledged this to each other, before she ended things with her ex and we got together several days after.

Her and her ex are in the same friendship group in university, it is how they met and she is the kind of person who doesn't like cutting people off and appreciates that the problems in the relationship meant that they weren't meant to be in a relationship together but rather revert back to being friends. I however, have only been in that one long term relationship, after which we maintained the strict no contact after breaking up due to long disfance 1.5 years into being long distance.

At the start of our relationship, I acknowledged to her that her messaging her ex as friends was still getting to me a bit, she said that she would try keep conversations short, however if he messaged anything that required a reply, she would still reply out of politeness. There were a few small arguments when I would out of the corner of my eye notice her messaging him, I would get obviously 'off', she would question why I was off and know that it was because of him, but then reinforced that he just replied to a Snapchat story, and now the conversation moved to catching up about how family is getting on etc. This culminated about a month in, when my anxiety got quite bad and I made the shit decision to look at her phone whilst she was in the shower, I know this is completely over the line however I also feel I wasn't thinking logically at that point and driven more by anxiety. I found that she had been messaging that day, and because he had covid, when she went shopping, she bought a tub of ice cream for him and dropped it round to him.

She returned from the shower, and because I knew this, but at the time knew what I had done, reading messages, was awful, I merely asked, how's your ex doing with covid, to which she replied, she doesn't really know. She noticed the odd question and asked 'why? Have you been reading my messages?', I replied no. The next day, I messaged my gf admitting to what I had done, apologised profusely but also asked why wasn't she honest about how her ex was doing with covid or tell me that she went round and dropped ice cream off (which in my head is a bit crossing boundary when she knows I am anxious about her messaging her ex anyway). She said that she didn't think it was over the line, understood how it can make me feel a bit shit and we spoke about how I looked through her phone, at which point i promised never to do it again, I was sorry and we moved on.

We had a couple small arguments since this, when I once again noticed notifications pop up from her ex messaging her and would be visibly off, and a few weeks after, she messaged her ex telling him they can't private message because it was getting to me a lot. She messaged because I would get really anxious each time I noticed, but I also said to her that I hate being the one that says 'it's me or your ex'. I agreed that the fact that she's in group chats with the friends group and him is okay, and we have moved on from this bumpy phase and I have felt quite secure. 

3 months later, university is coming back in and I know in my head that her ex is coming back from home. I know that the friends group are going to continue to meet up and my gf will go (of which I have had no problem with), however I am getting really anxious over the fact that her ex will be at anything the friend group does too. I trust her, and I tried explaining this in my insecurity over her messaging her ex before, however she thinks it's a trust issue. I feel like it is more an anxious based problem, as I'm not afraid of her cheating, I'm afraid of her kind of falling back in love with him, or noticing he makes her laugh more. 

I have also been at parties with my mutual friend group with her and got anxious when I notice that she spent a lot of a party around one of my best friends and see her laughing at stuff he says more than stuff I say. But she reassured me it was just because he was chilling on the sofa not dancing, and she was tired so wanted to sit down. But I still get anxious because of this. 

I'm just looking for advice or if anyone else has had similar, I really do want to get to the point for her sake and for mine, that when she goes to a party or does something social with her ex and the group, it doesn't phase me. I hate the fact that it gets to me, but I also feel that I can't help that it does by just repeating to myself 'I trust her', because inside me, it doesn't feel like a trust issue, but more that I just anxious.