It's very natural to go through phases like that. Enthusiasm goes in cycles like anything else. There are times when you just have to make yourself do it because of your commitment, not because you want to do it. That's what makes the difference between success and failure -- the willingness to do what we need to do even when we'd rather not.
I really wanted to keep my passion and enthusiasm for as long as possible, and I have, but it wavers at times. This week, for example, I'm REALLY tired (I think it's just because of my TOM). Even though I had cramps last night, I did two classes at the gym back to back and worked my butt off. I wouldn't say I WANTED to, exactly, but when I did it I was glad I did.
One thing that has really worked for me is finding the activity that I really do have passion for, which is running. You may find that you want to try different forms of exercise until you find one that you absolutely love, and then you won't feel like you're forcing yourself to do it on those tough days. But we'll always have tough days, unfortunately -- such is life. Luckily we have a place to talk about it, here at 3FC!