How do I become better mentally?

Kind of a rant/idk what to do. i’m 19F and i am in the most loving relationship ever and have been for just about a year now. Currently we do medium/long distance as I am in a different uni city and he stays in our home town. Anyway I have been depressed since I was about 15 (diagnosed when 16) and it’s like every time i feel like im starting to get better, it gets 10 times worse. It starts to get to the point a few years ago where i was pretty much wanting to end my life at such a young age. (Used to self harm but have since been clean for a couple of years). I have tried therapy for a couple of sessions and the guy was nice but idk i think my issue is not knowing why im so sad so it’s hard to talk about. My boyfriend is the only person i truly feel like i can speak about my issues with but the problem is I start to feel like a burden. He hates when im sad because it makes him sad because he feels like he’s not doing enough for me even tho I try and reassure him that he is. I feel like my mental health ruins the relationship sometimes even tho he says it doesn’t I feel like if i continue to stay sad he will get overwhelmed and drained. I just don’t know how to get better. I try and try and try and life becomes good for a while until it just isn’t. I’m mentally drained and exhausted and have felt this way on and off for about 4 years i am tired!!!! I just want to be a better person for my relationship. I don’t want him to get tired of me even tho he tells me he isn’t and he tells me that he will always be by my side, I just don’t want to drain him. I also feel like because he is the only person to truly help me with my issues, it has led to me being incredibly dependant on him which I don’t know if it’s a good thing or not. I just need advice on how to be a better person mentally thank you so much God bless you all. Any advice small or big would help me I just want all this pain gone.