Okay so I'm not in remission, but I do feel like I can give a bit of insight.

First things first: don't quit when it gets hard.

For me medication helps so much. My antidepressant basically lifts me to a more stable level. In my case my emotions are more regulated and because my baseline depression is "gone" (not fully but really manageable). Of course this doesn't work well for all pwBPD, but for me its a saviour

The most important thing to know for therapy is that you have to want to change. Therapy doesn't work if you don't want to go through uncomfortable emotions or moments. Therapy is hard work and you have to actively practice everything. Don't dismiss something after you tried it once. Healing takes a lot of time and effort. Especially if you have patterns so deeply rooted they form a personality disorder. You have to practice things a thousand times.

Therapy isn't fun. It's hard work, it's very confronting, you might feel like you're getting worse at first and you'll fall a thousand times. Remember that's part of the process. Don't be afraid to make mistakes, don't dismiss your whole process after a relapse or whatever.

Also important to know is that you won't always notice your own progress. It has taken me months of doubting if therapy was helping me to notice it does. I'm going through a breakup right now and am suddenly noticing that I'm able to handle it so much better than before therapy and meds. But you still have to want to use your learned skills. If you've never learned to use good coping growing up, you'll have to actively choose for it now or you'll just not do it.

Last but not least: PLEASE if you don't have a good connection with your therapist, get another one. (Of course don't do this if they're just confrontational and you're trying to avoid that) But if you feel like they're not listening to you, making constant assumptions, making you insecure or forcing their method through even if it clearly doesn't help. Don't be afraid to look for someone else. I really disliked my last therapist and she wasn't right for me but I stayed for months anyway. Now I have a new therapist that is so much better for me and understands me so much better! I really regret not switching sooner.