Yeah I get pretty bad in romantic relationships it's a mess!! I'm 28 now and I have been going through it for years I've surprisingly had the same boyfriend for 2 1/2 years now and that is a long time looking at my dating record, I always felt it was easier to have a one night stand than to have regular sex in a relationship.
I still deal with this issue I mean I don't cheat on my boyfriend or want other men but I find it difficult on how to approach him for sex, at this point I only can if I had a drink or two to chill out. We go to couples therapy but I have a lot of issues we haven't even gotten to this part yet

I have bpd that is our main focus. Anyway I deal with it by going to psychotherapy DBT and meds.
I'm not saying you have bpd or need meds!! I'm just giving you my story and know where you are coming from it is very frustrating and hard to go through but therapy is always a good try if you find it's bothering you for prolonged periods.
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Just keep swimming
I have BPD or Autism or both, we may never know, the focus is always the symptoms, not the diagnosis