I've had several long term relationships and am currently in one. I'm also not monogamous so the approach I take is a little bit different.
My therapist has been really helpful in helping me with my own internal monologues, but what really helps is having a partner who can accept all of me, paranoia included. What I love about my domestic partner is that no matter what, I can talk to him if I'm scared. And then he talks to me when he's scared. I feel like we both have our own separate problems and issues and we really help each other out.
Having a therapist is a really crucial thing. I do think a lot of people end up treating their partners as therapists and that's where it becomes a problem. If I feel like we're having a communication problem, we'll talk to my therapist or my partner's therapist together.
I think over time I've been able to grow and have faith that my partner loves me and wants to be with me. But it also helps that my partner reaffirms this to me every day. When I feel scared or worried, I actually try to write little cards to my partner saying how much I love him and how much he means to me and it helps.
It's HARD for me to be vulnerable, and he knows this. He's very good at reading me when I'm clearly unhappy but feel scared to say anything. We've had some situations where I've been too afraid to ask for what I want and have been upset when things happened, and he's just asked me for what I wanted and helped. Like one time I really needed him because I had been sad all day and I tried to get him to have dinner with me before going out but he didn't really want to. I got a little upset and then VERY upset because I just didn't know how to ask and then he just said, "Do you need me to be there for you?" and I said "Yes" and he came right over.
I think partners can really help by understanding that you're afraid of vulnerability. Over time my domestic partner is learning more about what being a child of a BPD parent can do to you and he tries to see how he can help me.
But I don't know if this is about relationships so much as about your own personal growth, and that just takes time. <333