I've been in a similar situation - my one and only bisexual experience and relationship. To make it worse, she was Bipolar, and was probably too at the time, just not yet diagnosed.
I'm not sure if it's especially lesbian relationships that land up being so intense and a rollercoaster ride, but I definitely agree.
I ended up in major depression and finally went to see a pdoc. When our relationship finally ended I couldn't take it, and tried to OD - I felt no purpose.
I really do understand, but some times you need to realise that not all things that are nice are also good for you.

You need to realise how bad this relationship is, especially as it is really hurting your emotional state. This kind of relationship is toxic and you deserve to be treated better. You don't deserve to constantly play second fiddle.
I'd cut all ties and find someone who really loves me back the way I love them.