I'm in a similar situation in that I started playing again after a very long layoff. When I first started back up, I had the exact same problem on my serve. I couldn't keep my right hip/leg from rotating forward and coming down first.
I took a lesson and the instructor had me work on starting my stance with my right leg much further back than normal, almost ridiculously so. As I went up to toss the ball, my right foot would slide forward into a more traditional position, and as I began my serve I'd concentrate on kicking the right leg back instead of following through onto it.
It felt really awkward at first, but it helped a lot, if for no other reason than it slowed down my whole service motion and made me focus on what I was doing. Now that I'm more comfortable with the proper movement, I don't have to start with such an exaggerated stance anymore.