I don't really expect anyone to have any advice except to tell me to chill out, but I need to vent and don't have a support system.

I started dating my FP a few months ago. I'd say 80% of the time it's great and I feel so loved and understood by them, especially because I've been able to be honest about my BPD and they never judge me or make me feel bad about it. But 20% of the time, I feel like I'm not enough and my intense fear of abandonment rears its ugly head.

My partner is very social and likes talking with new people, going out all the time, etc. About two weeks ago, I found out in a roundabout way that they were using Tinder to talk to people since they couldn't go out because of quarantine lockdown. We're in a monogamous relationship, so it obviously really upset me. I talked with them about it and they reassured me they were only doing it because they wanted to talk to new people. They had no intention of trying to hide it from me, they just didn't think it was worth bringing up. And that they only ended up talking to one girl and it was in a totally non-romantic, non-sexual way. (I asked if they could show me a screenshot and they did and it was just them talking about music). They also said they have been up front with her about having a partner. I tried to stay level-headed and said we should discuss if they are having second thoughts and would prefer to have a polyamorous relationship. They said no, they only want to be with me. They also deleted their Tinder after I told them how it made me feel, without me asking them to.

And yet despite their reassurance, I just can't shake how upset it's made me. Since finding out, it's made me even more hypervigilant than usual and I keep seeing what I think are signs that they don't really love me and are going to replace me with this girl the moment my mental illness becomes too much for them. Because of that, I've been holding back talking about it any further... at least until this past weekend when the paranoia remerged and has been consuming me since. I asked if we could talk on the phone tonight because I have some questions about the situation and our relationship that have been weighing on me. But I feel like bringing it up again is going to push them away from me. 

I feel so conflicted. I know a big part of this is my BPD, but I also feel like this is a weird situation that would make even "healthy" non-BPD people feel uneasy. I can understand needing connections and socialization outside of just me, but to me, doing it through Tinder is what feels wrong and slightly disrespectful since we are monogamous. I also don't understand why a single woman would want to talk to someone in a monog relationship, especially if they met through Tinder where the underlying insinuation is that people are on there to date or have sex.

I wish I could let it go, because I have more reasons to believe they love me than I do that they are unhappy and looking to cheat or leave me. But no matter how hard I try to push it down and ignore it, it's there in my head everytime we're together and it's making me feel resentful and like I need to push them away to protect myself.