Struggling with a Situationship and Fear of Losing My Best Friend

my ex, and I have been in a Situationship for almost 3 years. he's my favorite person and honestly the only person in my life at all. every time he's tried to end things I will talk him out of it. tell him how much I need him and how hard I'm trying. I tell him how worried about him I am because he only does this when he's in some type of self-destructive mood. I can tell because of the negative self talk that he uses to justify leaving me. he'll say things like I'm a bad person you deserve someone better im holding you back etc. he's undiagnosed, but I do think he has borderline too or some thing similar. Maybe he just falls victim to seasonal depression very easily because It seems every time he's pushed me away it's when it starts to get cold outside. these self-destructive episodes scare me so much because I used to go through it and not even know what was going on at least now whenever I have a manic episode, I can tell. from what he's told me, I know people have abandoned him because of his depression. this is the reason every time he tries to push me away I have to talk him down. i can't abandon him he deserves so much more than that and I know that I can't just magically cure whatever it is that plagues his mental health but when someone you love tells you that they are awful and they deserve to be alone how can you just give them what they want and let them think that they are right? last night he ended things for real this time. he didn't say any of the usual things that I can refute like the negative self talk. he just simply told me that he loves me, but he's not in love with me and that I deserve someone who could love me to the full degree. How are you supposed to argue with that? we stayed on the phone all night, talking crying apologizing to each other for all the mistakes we ever made. I told him I was sorry that I couldn't let go and that I dragged out the pain so much and he said it wasn't my fault because he let me and he really wanted things to work too. he promised me he was going to get help because he knows he has a pattern. He told me maybe if he has "a clear mind one day..." and then didn't finish the sentence. I don't wanna get my hopes up, but I can't think about anything else. he also said that I am his best friend and he never wanted me to not be in his life. I just don't know how logistically that will work. he is and has always been my best friend but we've never been just that. The majority of our relationship consisted of cuddling in his bed watching Netflix and sex obviously. it was already so rare for us to go on dates and do normal little activities. I just can't see him meeting me at the mall for a friendly excursion. I'm terrified that I'll never see him again. We'll just talk via text until it dwindles down and we grow apart. this man is the center of my universe, and I have just been thrown out of orbit.