You are not alone in how you feel, though I was wondering if you have noticed a pattern with when the black dog strikes..... because this sounds like me, What triggers it off? Are you giving yourself time to 'grieve' or cope with the downside of things. Are you managing to separate the issues of 'your problems' and 'their problems' when things don't work out. There are just some things we don't have much control over i.e. 'their problems'.
I understand the up and down emotions, and how quickly things can change with self-esteem and confidence, etc. I often just want to keep to myself, and family/friends, work and a small amount of sport keep me somewhat motivated to carry on. I am going through a difficult time in life as my son very recently passed away and the grief is all too intense sometimes which isn't helping, but things are starting to look brighter as the days go by, slowly but surely, though oscillating between good and bad days/moments very often.
Do you have a sport/hobby to keep yourself occupied? You need to feed your soul, and do things that make you happy. I believe that the 'regular' things help a lot. And having little goals to achieve in the short term can help, too. Have you considered shared accommodation?
Although I have mild depression myself, I refuse to go down the medication path, but counselling is helping. Don't give up on that therapy if it is still available to you. You will find someone who you can click with. As for friends, they can come and go, but the few you can really count on might just need that added push to understand what you are going through. What are their coping mechanisms?
I hope your days ahead cheer up and things look brighter for you. You are not alone. X