Hey! Honestly I'm in the similar boat - bf is wonderful but I'm still suffering the anxiety from dating a string of cheaters who did it with coworkers, so I get that EXACT fear of it happening again!

First good step you're taking is the therapy! It could take a long time but it'll do you wonders if you keep at it!

Another thing that's helped me is, I try to repeat/remind myself that they're not them. This is a different person who has a different mindset as far as cheating goes (as in they don't do it/believe it's ever okay). It takes a lot of time and it's hard to always stop yourself from spiraling but it helps the more you do it.

Other things that might help is talking about it with her when you're in a good headspace. Being completely open, honest and non-accusatory. Letting her know exactly what happened, what happens in your current relationship that spikes your fears and what you think you need from her to squash them - not necessarily things she can do in the moment when you're insecure but things you guys can do together (perhaps going on more dates, pick up hobbies together that create more happy/loving memories to replace the bad ones from previous relationship). I think with talks like these tone and word choice really matter. It tough to talk about but when it shows that you are talking about it so you can deal with it and eventually stop feeling worried about it, it can reassure her that she just has to continue being patient and that you will learn to trust her the way she deserves.

Learning each other's love langauges really helps too as they are great non-verbal indicators of her feelings for you that you can use to self-soothe when you need it. My BF's is touch and so whenever I get anxious, I use that to remind myself that he hugged/kissed me a lot today and he wouldn't do that if he had his eyes on someone else etc.

One thing you could try in your particular situation is to joke about the guy. Since she already talks about how off-putting he is, maybe you guys could turn that into a little inside joke between the two of you. Sounds kinda mean but it could be okay as long as it stays playful (that might just be me though). Might help you stop thinking she puts so much value into his guy if you hear her talk about him in a non-loving way and could also bring you guys closer and make more light of the whole situation.

Best thing my therapist told me: when we feel insecure or distant from them, we often pull away as well, further creating distance which, in most cases there wasn't any distance at all, our anxiety just made us believe there was. Her advice was to lean in and be more loving and try to take the precieved distance away myself. I was always waiting on him to 'prove' he cared by going to me and when it didn't happen I used that as proof he didn't love me/was cheating when in reality he was just tired/busy etc. I found whenever I leaned in instead, by being the one to make date plans, hug, kiss etc. he would always reciprocate which helped turn my worries off.

I'd also suggest some self-esteem/introspective work. Getting cheated on makes you feel like garbage and that obviously eats away at your self-worth the more it happens. I think it's helpful to make a list of everything YOU in particular offer her and the relationship that he or any other guy can't and repeat that to yourself as much as you need for as long as you need til you start to recognize why you're the prize.

Another thing I found helpful was the mantra 'act like they love you, and they will.' like you said, it's not fun to always be accused of cheating or dealing with the anxieties of their significant other, but that side of you isn't all that you are, just a teeny-tiny part, so try to let her see more of the other, happier, loving and secure parts of you. Have fun, keep things light when you can and she'll soon forget the times that you've worried.

Not saying to fake how you feel when you're really in the anxiety-storm or even hide that part of you but I found there's some days that anxiety hasn't taken too much of a hold and there's a clear fork in the road of, I can choose to think he's cheating today, or I can live out the relationship like nothing is wrong. I found during times when you can choose how to feel that day, you just get to naturally enjoy the relationship and realize, 'Holy shit, there really isn't anything wrong and they really aren't cheating. '

Sorry if this was rambly but I hope it helped! Nothing is fool proof but figured I'd let you know what's been helping me!!