Oh dear, I really feel for you.
I think the first thing to do is go back to your GP and change your medication as the one you're on isn't working.
Don't feel guilty at all about 'breaking her heart'. 2 years is a fairly standard age gap between siblings. Unfortnately, your DD isn't dealing with it very well but it's still a behvioural issue on her part and should be dealt with. I think it's worth untangling the problems and havign a set strategy for each, along with an overall theme of getting her used to not having your full attention.
Bedtimes - It's back to sleep training. Does she have the same bedtime routine as before your DS was born? If not, try getting back to that routine. If it was always you putting her to bed before, can you take responsibility for the last part fo the routine e.g. reading a story together and getting into bed? If she makes a fuss, reassure her briefly - 'Goodnight DD, it's sleep time now. I love you.' - but then leave and ignore. Repeat. If she's coming out of her bedroom, take her back to bed without fuss and/or consider consider putting up a baby gate.
Hitting - completely unacceptable and you need to put your foot down about that. The strategy you use is up to you but it she needs to be 100% clear that she is not allowed to hit anyone. Time out might work or the Supernanny-style naughty step. Make sure she has a cuddle when she's calmed down!
Respect for your DH - it must be really upsetting for him that your DD is acting like this but try to help him not take it personally. If it's physically possible, get him to take your DD to nursery. Can he take her out for 'treats' like going to the park, swimming etc.?
Is she happy once she's at nursery? If so and if you can afford it, I would see if she could go back to doing 2 days, to give you both a rest. Nursery is one environment that hasn't changed for her, whereas at home with you has changed completely and needs some getting used to.
Does she help out with her baby brother? Could you give her some age-appropriate responsibilities so that she feels involved? Have you tried putting your DS in a sling so that your hands are free to do craft activities/baking/stories with your DD? It's probably the last thing you feel like doing but spending time on a focused activity with her could help her feel more secure. Otherwise, are you getting out at all? Can you take her to the park, a story session at your local library or soft play?
How is your DS sleeping? Does he disturb your DD at all during the night? Is there anything you can do if he is?
Are you planning a holiday this summer? If so, it could help you to spend time as a family and for your DD to do fun things with your DH with you there too. If not, can you do a 'home holiday' with family activities for a week?
This must be really hard for you and I hope your DD settles down soon. I think the most important thing is that she is reassured that her baby brother has not replaced her but that she needs to learn to share you. Making sure she has stable routines is easier said than done but I'm sure it would help her to rebuild her sense of security.