"Once you get depressed, you don't really feel like doing anything," Federline, 31, tells Access Hollywood. "You're kind of discouraged about yourself and then the weight gain too, or that makes me more depressed. … I mean, it's a combination of all of those things."
The difference for Federline was that his troubles played out on a public stage.
"Everybody knows what happened with me and Brit," he says. "I'm not going to say that's the total cause of it but, I mean, just not being happy with myself was probably the main part of my depression."
Now slimmed down thanks to his work on VH1's Celebrity Fit Club: Boot Camp, Federline says his life is turning around, and that he now can be friends with Spears, 28, with whom he has two boys.
"I think as time goes on, it keeps getting better and better and better," he tells Access Hollywood. "I mean, I'm really happy the way that my life is going right now, I really am. I think, you know, I have come a long way. All of us have…my whole family. It's gotten so much better from where we were."
With Spears touring again and her affairs under the control of her father, Federline says, "I'm really happy for her. I'm very happy for her."