What does/did your training schedule look like before you started feeling this way. It sounds like a classic example of overtraining, but could be other reasons. That's why I asked about an example of your typical training.
I was a rower, and I fell into overtraining very badly. When I had started, I was in very good shape and strong. I picked up the rowing fairly easily and began racing pretty quickly. Because of my competitive nature and immediately getting into the racing I had taken on a heavy training schedule that eventually took it's toll on my body. I still have issues with tendonitis in my forearms near my shoulders. Too much, too soon, or just too much period will eventually break your body down. Your training schedule must have designed rest days and easy/hard days mixed. The Chris Charmichael book discusses overtraining, and he states that many people are often amazed at how "easy" the training feels to some of them that take on his coaching programs. He also stated that rarely has any amateur rider every come to them with their own training program that was too easy. Usually they are doing way too much or going way too hard.
The problem with most athletes, especially those that have a competitive nature about them, is that if performance doesn't increase or they seem to be dropping off they think that they need to work harder. Often that makes the problem of overtraining that much worse.
I experienced severe overtraining and vowed to never go through that again. I've just taken up riding and worked up a program that will slowly give me the gains I'm looking for. I have to continuously monitor how my body is recovering and force myself to stick to the program even when I feel like I want to do more.
I understand too how it mentally affects you when you are overtrained. It's just as bad as the physical side of things. It really gets you down and you can mentally feel like total crap about yourself, your performance and the general outlook on things daily.
Hope this helps, and I highly recommend the Carmichael book. There is also another site that talks about overtraining well. I can't remember right now the exact address, but you can probably find it by with a google search on "Coach Carl bike training". Have not read his book, but he has free articles that will give some insight.
Sorry this was so long, but after having dealt with this myself, I hate to see others going through the same downward spiral that seems like it has no way out.