Unfounded insecurities of a BPD brain

My girlfriend, (E) and I have only been together for 2 months and we are off to a rocky start. We met once back in 2015 under some rather unsavoury circumstances but remained distant friends throughout the years, with on/off contact. We got back in regular contact again in September 2019 and supported each other as we had both recently been through breakups and were struggling. Quite quickly, feelings started to resurface/form and the situation we found ourselves in became very confusing for us both. Her breakup was much more recent than mine, so she was still in the midst of dealing with her emotions (whilst also still talking to her ex - which is still true today).

As our feelings towards each other grew, I started to get hurt when she would talk to me about her ex, although I tried not to show it, she knew that it was hurting me. We ended up having a very painful conversation and she said that she was confused and didn't know what she wanted. That she doesn't want to hurt me and can't deal with the responsibility of somebody else's emotions right now as she needs to focus on herself. All very fair, I completely understood what she was saying, but the way she put it was so cold. I know the only reason it was cold was that, in that moment, she had to just dissociate so she didn't become overwhelmed herself, but it did really hurt. After we finished though, she changed her mind and said that she wanted to carry on how we were (which was still pretty casual).

So we carry on seeing each other and it's mostly okay. She's still talking to me about her ex but I decided that I'm going to try not let it bother me. I started off as a friend she could confide in, and before anything else, I want to be there for her and help her however I can. And it genuinely did stop bothering me for a good while.... Then on Christmas, she asked me officially in the most romantic way possible. I said yes and we have been together since then. However, over the last month or so, we have had some similar moments where she has said that she's unsure of what she wants. She has her own struggles with mental health and her identity so I know that plays a huge factor in it but it's becoming increasingly difficult to not take it personally. I know a lot of it comes from her feeling inadequate/insecure which is completely my fault because she does everything to always assure me that she loves me and not her ex, but there's like this overwhelming loud voice in my head that tells me she would rather be with him. She's told me before that she feels like nothing she does will ever assure me and that it makes her feel like she's not good enough, which I fucking hate because she is more than good enough and I hate myself for allowing her to feel less than that.

Every single day, no matter what we are doing, I'll find a way to relate it back to her ex and I can't even properly enjoy our time together because I'm just constantly thinking about her with him or how he would probably be better for her and more fun/have more of a connection with him. (I do this in my head, not verbally) I know this is all completely ridiculous and unfounded. He was by no means a bad person, but he did do some pretty awful and thoughtless things whilst they were together so I know my insecurities are completely fabricated by my own mind but I just can't seem to escape it. I don't ever tell her these things because I don't want to make her feel bad and I don't want to ruin our time together but in my mind, I'm battling every day and I'm just exhausted.

We have spoken about it twice in the last 2 weeks and I let it all out and she was extremely understanding and actually very helpful and reassuring. But I don't want to have to keep relying on her to reassure me all the time, she shouldn't have to. She does still talk to her ex and although it bothers me, I don't ever want to stop her doing so. Their relationship was complicated and he was messed up so I understand why she feels a responsibility for him, especially as I know she got insense with him after he broke up with her. I 100% trust her not to cheat in any way, so that's not the issue. The issue is completely me and my mind imagining scenarios or conversations they might be having (not necessarily cheating, just things that would hurt if I read them). I just can't escape my own head at the moment and I really just want to stop being so insecure.