Ok, I don't think I'm doing this right ):

Think of it as just part of your basic body maintenance. You wouldn't stop bathing or brushing your teeth because you lost motivation. Don't think of it as "Ugh... I have to exercise because I hate my belly." That's punishing yourself, and no one likes to be punished. Except, apparently, the chick in 50 Shades of Grey.

Change you focus to "I'm doing this because I love my body and want it to be strong and beautiful." What worked for me was setting a personal challenge to be able to run a 5k. I was never, ever athletic, but my brother and his daughter were, and I realized, "Hey, genetically, we're about the same. There's no reason I can't do that!" So I started training. Every week, when I could run a little further or a little faster, it was a victory that kept me motivated.

I'll be honest, I still don't particularly enjoy weight lifting, but it's something I do three times a week because it does amazing things for my body. And it's really nice being strong and not being as accident and injury prone as I used to be.