Is there anything I can do to improve my relationship or should I reconsider it entirely?

I have been dating my boyfriend for almost two years now and we love each other very much. He’s my first boyfriend but we’ve been through a lot together and I’ve never felt closer to anyone in my life. He has some mental struggles he’s been working on and that I try my best to support him through, but now he lives two hours away from home and over three hours from my college campus so I can’t be there to support him in person as often as I’d like. It doesn’t help that he can’t drive so I’m the one who has to make the trip almost every single time (even with a full time summer job and summer classes, neither of which he has) unless he happens to be in town for other reasons, but I’ve gotten used to that. It’s just the bad days recently are really bad and whenever they happen I’m tempted to end the relationship right there. Whenever he’s having a bad day he usually starts texting these vague, concerning statements without actually telling me what’s going on and when I try to reassure or advise or just be there for him he goes off the grid without a word, sometimes for days at a time and I’m left to sit and wait and hope he comes back alright, feeling like a failure for not being able to help when he came to me in distress. I feel pushed to the side. I realize he has to work through his own problems and they’re not mine to solve, but I’d appreciate some more communication and some sensitivity for how it makes me feel when he does that. But if I try to mention how I feel after the fact he feels as though I’m making the situation about me when he was the one having a bad day and I feel extremely guilty. He also mentions on occasion that I’m one of the only people keeping him going in life, that he’ll be okay as long as I’m around, and even though I never had any intention of leaving the relationship it makes me feel trapped or guilt-tripped sometimes, even if it’s unintentional. And I feel like I can’t bring these things up on a good day because it’ll put him in a bad mood again and he’ll just feel bad about himself without actually fixing anything. I also feel as though I’m expected to move on as soon as he does and things never get talked about. He always comes back and apologizes for his behavior but the pattern never changes and we move on as though nothing happened. I feel like I’m the one taking care of him all of the time, and he says he’ll always be there for me too but I tend to hide my emotions because I don’t want to become a burden (a habit I know I need to break) so he hardly ever has to be, making this all feel very one-sided. I feel like I only exist when it’s convenient for him. If he’s not up for having a girlfriend some days he just won’t talk to me until he is again. I hate how that feels like I’m some sort of part-time thing he can choose to interact with on some days and not others and I get no say. There was one incident where I was having some panic attacks and asked him for help or comfort and he offered to call since he knows I like to talk with him over the phone (something we rarely do because he prefers text). While over the phone, I was making conversation to distract myself and brought up an apparently touchy subject for him (I asked how his progress towards getting his driver’s license was going) and he shut down, said he was tired and that he wanted to go. I apologized for bringing it up and asked if he would just stay for two more minutes since I was still panicking and didn’t want to be alone but he hung up anyway. I felt awful. He then texted me saying he felt invalidated and asked why I would manipulate him into staying with me on the phone, essentially making me feel worse than if I had never told him what I was going through in the first place. He also brought up how it feels like he has to take all of the blame all of the time and that it’s unfair, but I feel like the problems are usually coming from his end. But I apologize anyway whether I messed something up or not to somehow even things out for him, which I don’t think is very fair either. It later got resolved and he once again apologized and was actually there for me the next day, staying with me on the phone while I was crying on the bathroom floor and he helped me feel a lot better to know he was there. That’s the confusing thing of it all, I feel all these negative emotions on the bad days, which happen maybe a few times a month, but as soon as I’m able to see him in person and spend time with him or whenever it feels like he’s putting actual effort into our relationship everything feels perfect again and we always have a wonderful time. He’s my best friend, I really do love him deeply and feel like I’ve bonded with him unlike anyone before and he says he feels the same. We talk and laugh about our future together without a care in the world. I’m sorry for the novel I’ve written here, but I just can’t tell if this is simply the stress of a long distance relationship that I need to be patient with until it resolves with time, if I’m at all the cause of this, or if there is a different fundamental flaw that needs fixing. I’d prefer it if breaking up with him was an absolute last resort, I really don’t want to abandon him, but any advice would be greatly appreciated.