I am sorry you are feeling so down. I have not been treated for depression so you can ignore everything I say if you like and I am sure you will get much better advice from others on this site.
However, here goes, it sounds like you have a lot of stressful situations to deal with at the moment: you have to cope for your mum, you are still deeply grieving over the death of your sister, you are frustrated over the lack of progress with the doctor and you are physically struggling too. i think that you are being hard on yourself thinking that you are slipping backwards. Anyone with all these stresses to deal with would be struggling. Also, I too avoid contact with people I know when I feel stressed because they always seem to ask about what is stressful to talk about and I can't deal with it. I also tried the spending my savings on holidays that left me poorer but no better emotionally and when I went on my own it was worse than ever. I hated it. Be sure it's what you need. Running didn't help me. Sorry I can't be more help.