Like the other poster said, try to keep the severity aspect in mind. Many people who have had such terrible experiences with BPD have known people whose disorder was very severe and who refused to seek or accept help for it. Regardless of the specific disorder, that's going to be emotionally taxing on anyone. The important thing is to at least try your best to keep your disorder in check, communicate with those around you about how it affects you, and most importantly, seek out professional help if you need it. There's absolutely no shame in going to therapy; if anything, it shows that you're capable of recognizing a problem and taking steps to fix it. If you can't afford professional help, that's okay too -- you can still do plenty of things on your own to work on controlling your disorder. (If you haven't already, I would recommend doing some research on Dialectic Behavioral Therapy [DBT]; I've been going through it for a few months now and I can already tell that it's helping me a lot.)

With your fiance specifically, I would recommend doing what I do with my SO: constantly update him on where you're at emotionally and never be afraid to explain something after the fact. There are plenty of days where my BPD turns me into a sullen, snappy, irritable mess, and no matter how hard I try, I can't snap out of it. Those are the days when I just have to say to my SO, "Hey, I'm not feeling very good today mentally. I'm sorry in advance if I'm short or snappy with you. I don't mean to be, and I'm going to try to avoid it. Just know that it's nothing personal." Then, if it does happen, he's much more patient and understanding about it. '

You can also message me if you ever want to talk/vent. I know how frustrating and scary it can be to deal with this.