oh gee, I can relate, although my situation is not quite so dramatic.
My ex has been very unbalanced at times and I have considered denying him access altogether and going to court, but all the legal aid people tell me that he would probably be awarded same amount of access as he currently has since no abuse of dd can be demonstrated. Actually, I don't think he does abuse her (although she's never very keen to go and stay with him) but he's certainly often horrible to me, and I feel awful letting dd go with him when he's in an obvious 'mood'.
I am in Australia, and here we have a 'default law' where primary caretaker (ie, mum) can deny access to other parent (ie, dad) until court has established that dad is a safe person for child to be with.
It seems you have already been through legal channels, but if there is a similar 'default' law in UK, and you are really worried, perhaps you could keep a record of all threats and incidents coming from your ex and his new girlfriend, deny access to ex if you have bad feeling about it, and use your journal of their bad behaviour as evidence in court? In Australia, such records do count and will go towards the judicial decision vis-a-vis access.
As for anti-ds....if the one you are on is not working, try another. Good old fluoxetine (Prozac) works for me, but you need to start on a low dose and work up to a full dose gradually, especially if you are prone to anxiety. You may also require short-term script for tranquillisers such as Valium when starting anti-ds, to mitigate nasty side-effects (but only use for short term, they are very addictive).
I my experience of depression, which is extensive, it usually takes at least eight weeks for any SSRI such as Prozac or Cipramil to take effect. And initally one often feels worse. Hang in there! You are doing marvellously - it's great that you're at uni, and you sound like a great mother. Your child is very lucky indeed. Do you have support from a relative such as Mum? I managed to finish a postgrad degree shortly after leaving abusive ex and in the midst of depression, and I've got mum (who did a lot of baby sitting during days so I could work) to thank for it. If you don't have this option, perhaps you could look into daycare options at local women's shelters. I was housed in private low-rent accomodation by women's shelter after leaving ex, and they provided free daycare whenever I needed it. Even though I was not actually staying in the shelter. Most women's shelters are wonderful, and will offer counselling or daycare for women coming out of abusive relationships even if they are not actually staying in shelter (in Australia, anyway). It might be worth investigating.