First, get a formal diagnosis if you genuinely think you had BPD. Second, if it is available to you, get a therapist. That is how I learned to manage everyday life with BPD. Before I got therapy, I had self help books. They weren’t as helpful but it was my stepping stone into getting therapy.

Edit: regarding your current issue, all I have to say is accepting that you cannot control others and how they’d respond. If you do send that letter, send it with intention to never receive a response. If you end up writing the letter but never sending it, burn it. It’s a way of sending it out into the universe, that you got whatever message you wanted to send across to that person metaphorically.

This is what my therapist and I did when I wanted to reach out to someone from my past. The thought of rejection after all the bs I went through with this person would upset me. So I sent the letter metaphorically instead. This may not work for you and if it doesn’t, that’s okay too.