Stuck in a Plateau?

Hello all, So I've got a plateau problem that I've had for years now. I'm intent on overcoming it and getting faster, though I'm not sure what I"m doing wrong. I practice by myself because our masters swim times are WAY to early for me, (5:30 am). I have a feeling that my catch is pretty decent and my position in the water feels good, (I've been told I have a very efficient stroke) and I can reach dps of 12-13 strokes per 25 yards (I'm 5'4"). I swim a relatively stratight track and I have a good streamline because I used to be a diver. That being said, I'm VERY slow.. I average about 40 seconds (pushing off wall) for 50 yard freestyle. When I try to increase my stroke rate for sprints, I tend to keep the same times as when I do sets of stretch freestyle. Any ideas on what I might be able to do to increase my speed? I wish I had a video to show but alas no luck. Thank you for any tips you can give!!! -Jeanette