Struggling with an On and Off Relationship

I'm 18 and I'm not entirely sure what to do. I've been in a on and off relationship that is like my life. the girl I'm with has had alot of problems with her family and with mental health. in trying to help her i brought myself down as well. this is one of the key factors leading to my diagnosis of depression for the last 3 years. when I was at my lowest I went with one of her friends and then me and her broke up. after like 3 weeks me and her got back together and she cheated on me (I didn't find this out until 4 months later). she then proceeded to act like she's dating another guy, send nudes to another guy, and wanted to have sex with my best friend. i don't trust her anymore because of all this. I'm so worried about her talking to other guys. I've been giving this relationship all I have. recently I've felt like a second thought to her guy best friend who is gay. she's been spending all of her time with him and lying about the stuff they do together. when I ask my friends what to do they say I should get out of it but I don't want. I'm in love with her I want to be with but I don't know how much more of it I can take. she never listens to my feelings without getting mad about so I went to here. I'm looking for any advice some of you may have.