Trouble after anti depressants

Options
I really would like some input on this. I was taking anti depressants and beta blockers that caused me to gain 20lbs in 2-3 months. About 3 weeks ago my doctor told me this and my other problems were from my medication and I stop them all cold turkey. Ive been on a very intense diet for 2 months and I havent lost any weight.
How long does it take on average to get back to normal/get the meds out of your system and start losing?

I've been "underweight" all my life so this is very frustrating. I eat around 500 calories a day (please don't lecture on this, I have to because 1200-1400 is how I gained 20lbs in 2 months) and am now doing 2 hours of cardio a day.