Hi 1lookingforward,

You were going through a transitional phase in life, and feel like you were taken advtange of because you were stretched thin. I'm sorry.
She was in therapy 5 years and she said that she might be bi-polar? What sort of therapy?
I'm happy to hear that you are in T. You had a long going on and you lacked boundaries. You feel like you were blindsided for the lack of boundaries. Many members and their exes here didn't show obvious signs of a mental illness. It tends to happen later when we become intimate with our partners. The disorder is triggered by intimacy. I think that your being hard on yourself.
There's no right or wrong with writing the letter.
You want to explain things from your point of view and you may be disappointed when you don't get the reaction that you expect. It's going to invalidate your feelings if she doesn't respond or acknowledges that she has read it. She may also blame you for everything in the r/s as well. Realize that you may not get answers that you would like to hear or that your voice be heard.
Be mindful if you choose to tell her that you think that she's borderline. Your heart is in the right place. On the flip side,
her reality is real to her, just as your reality is to you. This is a
a part of her personality. If you were to put yourself in her shoes and were told that she has a serious disorder, you may really take offense to that. You're also running the risk of making her symptoms worse.
If you look at the lessons to the right "Choosing a Path" and Step 2. Take a step back and give yourself emotional and intellectual space. Look at the bigger picture before telling someone you care about that you think they are mentally ill. BPD is a difficult disorder and it's often misunderstood. The person that you care about has to want to be committed in doing the work in therapy. It may not work and if it does there's the possibility that it can take years. I hope that helps.