you would be changing the cause of the problem, you would be changing yourself and the way you process and cope with/learn to live with who you are and what has happened to you.
You are (obviously) not the worst mum in the world but part of the problem will be how hard you are on yourself for choosing 'bad' relationships/not being 'perfect' parent. With depression the biggest battle is looking after yourself, allowing your feelings, taking care of you before you try to take care of others, forgiving yourself quickly for 'mistakes', not carrying guilt.
Yes, of course I am going to say therapy will help (the right person whom you like and believe is supportive towards you) but I would say that as I am a therapist
Before I trained to be a therapist I had a terrible marriage, abusive and alcoholic parents and lots of death and loss. I had 4 years of therapy and was transformed by it. Enough that I am now able to look after others.
Talking on here really helps some people, as done calling Mind/Saneline/the Samaritans - i'm sorry you have so little real life support and have lost your best friend.
Try and give yourself credit for coming on here and asking for help