I am in a 15 year relationship with someone that I WISH was as understanding and caring as you. My husband wouldn't dream of posting here to get advice on supporting me. Your girlfriend is very lucky to have you! From your posts, it sounds like she may still be looking for the right med combination and a good therapist- both extremely important. Especially if she has mentioned suicide.
My advice to you would be to continue to encourage her to get well and find a dr. who can help. And remember that people with depression, anxiety, mood swings, can be very unpredictable. If you get upset when she is down, it may cause her to feel more depressed. Maybe when you see her in that state of mind, give her a big hug, tell her you care about her and you are here if she needs you and then give her some space. Take a walk or whatever you like to do and remind yourself that this is a real problem and that you are not the cause of it nor can you fix it. If she is open to the idea, maybe suggest she find a therapist and go a few times and then you go for an appointment too so that you can understand what she is going thru.
I hope that everything works out for the two of you. You sound like a great person! Keep us posted on how things are going.