You have to ask yourself some basic questions.

1) Perhaps you lost your enthusiasm for a good reason. Maybe your initial enthusiasm was naive. Maybe you liked mathematics for unsustainable reasons? You're likely to go through far larger "down" cycles in the future if you stay in mathematics (we all do, it's a chronic problem in the field) so if you're going to stick with mathematics you have to find some kind of joy you can hold on to, through all kinds of messy situations.

2) Maybe you really do love mathematics but there's aggravating factors causing problems. Maybe you're not working on enough easy problems. Solving easy problems is fun, and if they're the right kind of problems you build up new skills. This is one of the reasons why I frequent this webpage.

I had a bunch of issues like (2) as a grad student. IMO I'm mostly better off for them. I'm talking about issues like solving a problem (or making progress on a problem) and finding out perhaps way too late that the problem had been solved by someone else. I found it pretty tricky to balance focus with awareness of what other people are doing. The math arXiv and MathSciNet are excellent resources that help with that.

Being in a very active place where you can talk to lots of people about various areas of mathematics helps. Being surrounded by enthusiastic people helps. Going to small conferences where you get to know people can help. Talking to people about what you're interested in helps. Barring external impetus, "computing the daylights out of things" is an excellent fall-back procedure. I know quite a few very successful mathematicians for which this is one of the main approaches to things. You start piling up enough computations on things that interest you and you notice patterns -- maybe not what you were looking for, but sometimes of interest to people for reasons you never expected. Sometimes publishable. :)

edit: After reading your recent edit I can say I saw some similar things as a grad student. Sometimes the most talented/bright/whatever grad students have a hard time completing a Ph.D. Some students have too high expectations of themselves. They give up because they realize they're not going to prove the Riemann hypothesis -- they want that great big creative insight. In that regard it's good to ensure such grad students are working on both big hard problems and medium-sized publishable work, so that they can complete a Ph.D even if they never prove the Riemann hypothesis or whatever. Basically, always make sure you have a managable goal in sight. If your goals are only huge enormous things, you're setting yourself up for a potentially horrible failure. On the other hand, some people want that kind of situation, and if they're conscious of it, IMO you might as well let them be. It's their life. If they prove a major theorem, we're all the better off for it. If they don't, well at least they tried.

Discussion absolutely must happen on meta.– Ilya Nikokoshev Jan 2 '10 at 2:42