No I've never been with another borderline and can safely say I wouldn't want to be.

My previous relationship, minus the abuse toward the end and even his drug abuse, was very rollercoastery, addictive, and in hindsight actually pretty toxic. What I mistook for passion, was a toxic rollercoaster. Our fights ( on my end ) got uglier and uglier over the years as he eventually brought out the worst in me.

( It will become clear why I reference that crappy ex now ...)

My current bf? He's my anchor, when I'm attempting to capsize our ship, he's the calm when I'm a raging storm. He doesn't indulge my BPD *****fits and has actually learned how to diffuse them instead of fan the flames...

He's good at setting boundaries with / for me, which I (obviously) suck at. Boundaries that I need, and appreciate.

We balance eachother out, and our personalities compliment each other's, just like puzzle pieces that fit together, and I've learned that this balance, this opposite of me, is something I personally need in order to have a healthy, happy, sustainable relationship.

Also with him, I've learned that passion doesn't equal rollercoaster.

I don't need him to "get" me, he accepts me and loves me warts and all. My therapist "gets" me, you guys "get me" and this is all much more than I had ever hoped for in the past...

I have zero issues with expressing positive emotions, unless you count expressing them too much? Lol, but its something I've been working on. To not be overwhelming. It is however MUCH easier for me to write out my feelings as opposed to verbalizing them...

Maybe you could start with digital greeting cards? Find one that says how you feel and send it to her. Maybe searching through them will give you some inspiration. Not everyone is comfortable with expressing themselves verbally, words aren't the only language we speak. So find the medium that suites you, and I'm sure she'll appreciate the efforts you make at communicating your feelings for her.