Depression is ruining the time I have left with my boyfriend

So my boyfriend, josh and I were best friends for a little over 4 months, then dated for about 6 months 2 years ago, he was my first boyfriend I had that didn't cheat on me or treat me badly. We broke up because we fought all the time and I wasn't very affectionate and he was the clingy type. We were apart for about 9 months and not a day went by that I didn't think of him. I missed him every second of everyday thinking it'll get better and I was surrounded by guys that asked me out and wanted to go on dates but I never went. I kept telling myself hes not like josh. Well slowly we started becoming friends again because he wanted to stay close after we broke up but I just couldn't do it. Eventually I realized if he's in my life in that way it's better than not at all so slowly we got back together and things were amazing the second time around. I opened up so much he was still the same caring, gentleman I fell in love with and things have never been better. We've been together for almost 10 months and he leaves for college in a month. I was diagnosed with depression 2 months ago and now it's ruining my relationship. when I'm not with him, im sad and crying the majority of the day missing him, but when we are together I still find myself upset thinking it'll all be over soon and there's absolutely nothin I can do about it. I'm seeing a therapist which I like and im on medicine but nothing seems to be helping, we will be doing something fun and this wave of sadness and homesick just hits me. It's ruining what we have because I start crying and he can't understand why he gets frustrated because he doesn't know what I expect him to do. He's the boyfriend, he's suppose to make things better and im in love with him don't get me wrong, sometimes I just think of we broke up I could start getting over it. Start getting my life back but I know that's just the depression because Id hate myself everyday for losing him. He's the best thing that's ever happened to me and on top of all of this my brother leaves for college at the same time so I'm depressed 24/7 I can never get motivation to hangout with friends and all I ever want to do is be alone and cry or be with josh and try to not cry. It's terrible and its eating me alive knowing time is flying by and all he's going to remember is that when I had a chance to be with him I wasted it. Please help