OK tough love here then... 
Your Dr says you have low serotonin. So like a diabetic, you have to take meds, if you have a headache, or thyroid condition or whatever, you need treatment. Please take the tablets?
I have read in places that doctors are divided on the whole chemical nature of depression, whether it's an imbalance that causes the depressive thinking, or if the depressive thinking causes the imbalance. Either way the situation is the same, meds to stabilise, talking therapy, and ideally cognitive behavioural therapy, and time.
My X used to use the I'll leave you as his greatest weapon. Now I have said, Off you pop then, don't let the door slam you on the arse..
Once I decided that I'd had enough, that was it, decision made. Actually it was easy. The hard bit was sticking to it. He manipulated me like mad, but I stuck to my guns and he has gone. Thank. God. I worshipped this person, I thought he was my saviour, I thought he was a living god on earth. I longed to be considered his GF, and would have sold my soul to be with him.
Sadly, the phrase 'Be Careful what you wish for' fell on deaf ears.
I have really suffered these past 10 years, it has been real hell and 'Be Careful what you wish for' was on a frigging loop in my head for weeks at one point. That and the song Once in a Lifetime by Talking Heads. My nickname used to be Groundhog.... no prizes for guessing why!
I can tell you the hardest thing to do is to THINK about letting go, to make the decision. Once you have brought yourself into reality and realise that this is the only sensible option, that it has to be done, or indeed already IS done, everything gets a little easier.
When you are in an unhealthy relationship (and it certainly sounds like you were) you feel that the end of the world would be that he leaves, that you will crumble, you will be nothing and people will judge.
People we love are supposed to help make us feel better in some ways, they support us, care for us and make sure we know they love/care for them.
when someone starts lording stuff over you, when they threaten to leave you constantly, cheat, moan, get rough whatever they are NOT caring for us, and they are actually harming us. This guy I assume knows your background, knows how scared you are of being left, and THIS is the threat he selects to manipulate you? Bastard. For that alone, a swift kick in the balls tbh.
There is more hope for you than there is him. You can recover, you ARE a good person, he is a shit.
You may be down at the moment, but you can get better.
He will always be a shit.