Sorry if this is long and incoherent, please don't flame me, I feel so awful for getting to this point.
I have a 5 year old border collie who I bought with my ex when she was a puppy, we have had her since 8 weeks old, I'll call her K. My ex bought her from a friend of a friend, which now I know more about it I know was not the best idea as I think she was the runt of the litter and quite poorly socialised.
Nevertheless we worked really hard training her, taking her for long walks every day, meeting other dogs and she became a lovely well trained dog. She was always a little wary of other dogs, anxious and a little bouncy and prone to over-excitement, but I loved her immensely. She was originally meant to be ex's responsibility, as he was at home most of the time and my work became more demanding with full time hours. However, he just lost interest and refused to continue walking or training her. I carried on the walking and training for as long as I could, but then became pregnant unexpectedly and had hyperemesis and anaemia, I was still working full time and was just incapable of walking her as well.
During my pregnancy my ex became emotionally abusive, lots of shouting, a constant atmosphere and it was just miserable. K became more and more anxious, wary around my ex and protective of me. I eventually asked my ex to leave last Christmas- he took K with him. I reluctantly agreed as with a full time job and a toddler to look after I knew I wouldn't be able to give her the time she needs, he moved in with his dad who had a large garden, and they would both be at home all day.
Two months later I had a nervous breakdown and was very ill, I was almost admitted as an inpatient as I was so unwell. I still suffer very badly with anxiety and PTSD.
5 months ago my ex asked me if I would have K for a night as he wanted to go somewhere- he then refused to pick her up again, saying he wasn't having her back. 
She is even worse than before now- so anxious and unhappy, terrified of strangers and barks at everything. She shows me almost no affection, although she adores dd. And she has become incredibly fear aggressive, snapping or growling several times a day. I just don't think its a good idea having her in the house with a 2 year old. Its impossible to make K do anything she doesn't want to do, she just becomes a snarling snappy terrified mess.
I know I need to walk her more, but I just can't cope with it- she is so anxious around other dogs, her barking gives me extreme anxiety due to the PTSD and most days I am so anxious I struggle to go out at all, dog or not. I feel desperately sad that this whole thing seems to have broken my dog, I love her so much but she is just so unhappy. I know she needs a lot of love and work, and at the moment I am just too unwell. I have a new partner, but he works all the hours he can to keep things going. We have no money- my ex left me in an awful financial situation and the divorce is costing a fortune, so I cant pay for a dog walker or a behaviourist. Its just such a mess.
I think rehoming her is probably the best option- hopefully somebody else would be able to give her the time and experience needed to make her a happy dog again. But will anyone take her if she is so aggressive and difficult? I feel awful about the thought of giving her up, she is my responsibility and I just feel I have failed her. 
Does anyone have any thoughts?