I second the recommendation to watch the movie Fat Head! I think there is a lot of good info here.
1. Add fat back in your diet, esp. fish. I'd also suggest throwing in some coconut oil and butter from grass fed cows.
2. Fat is needed for hormone production. With out-of-wack hormones depression and other aliments can set in. Dr. Pritikin, who was Dr. Ornish's predecessor, committed suicide while promoting a low fat diet.
3. Protein is important for building lean muscle. If you don't consume protein your body will metabolize itself!
4. Dietary cholesterol is also important. Eat several eggs a day!
5. Seek emotional support! Internet forums are not enough.
6. If you're manic about gaining fat then make sure to not just weigh yourself but also measure your body and do body fat % calculations.
"Eat as if your life depends on it!"