<human>: *mandatory note that this is being typed on my phone.

Hey guys, I was hoping for a little help with an issue I have been having trouble communicating with my long term girlfriend.

Amy and I have dated for about four years now. Our relationship has had it's ups and downs as any long term relationship has, but for the most part we stuck it out and we both are pretty happy in our relationship.

Well, except when she gets incredibly depressed.

For the last few years I have been noticing her going through pretty big emotional swings. Sometimes she is happy and energetic and loving life... Other times she can barely make it out of bed, pushes me away and has even gone so far as to suggest to me she is worthless, and that I can do better. Generally the "up" period may last 3-4 weeks, with the down period lasting maybe 2 weeks. 

I love this girl very much, with all heart and it kills me when she pushes me away and goes down that rabbit hole. I have done research online and tried my best to be there for her in as best a way as a partner can. When she asks for time I give it to her, when she needs to rant, cry or laugh I do my best to listen, support or laugh.

Anyways, I guess what I am looking for help with is how to communicate with her that I think she needs help, more help then I can offer. Don't get me wrong I want to be there for her as best I can, but I'm in no way a professional, and to be honest the weight of it all is starting to stress me out and cause more friction in the relationship.

Thanks in advance for your comments.

Can you summarize the given text in a few sentences?
<bot>: LTR gf is depressed (maybe bipolar?) i want her to get help, but don't want to sound like I think she is a crazy person. Also mood swings causing friction in relationship.