Amanda I am so sorry for your loss and the terrible treatment you're receiving at the hands of your ex and his GF. Some people just lack basic empathy, huh?
It's a good thing that you're seeing your GP for this and anti-depressants could be just what you need to see you through this rough patch, but I would recommend some talking therapy too. It sounds like you're in a really bad place right now and a counsellor could help you through this.
When I was in a similar situation I got some counselling through my GP on the NHS and it really helped me heal without the possibility of developing a dependency on anti-depressants, which lets face it are really just a way to buy yourself some breathing space from grief and is not a way to heal.