How to make my hair grow longer?

Okay, so, I have this problem with my hair. I got it cut like 2 years ago into a bob, and in those two years, I trimmed the split ends every 1-2 months. I don't straighten it, and I wash it every couple of days. I've tried the just conditioning method, and that didn't work because it made my hair really greasy. I've had long hair before, so it's not that my hair can't grow long. I don't know what else to do! Please help! :)