When I'm pitching I always find myself spinning off to the 3rd base side (for a lefty). Is there a solution to this? I feel like this significantly decreases my velocity. Might this be because of a really tilted and over-the-top (like 12'o clock) release?

I can give you an idea because I've had trouble with spinning off in the past. Although video will help to truly diagnose the problem, you are most likely over-rotating either your top or lower half to generate more speed that your lower body should be producing either at the beginning through the alignment of the Force Vector or through 3X. The drive leg extends through the ankle and knee (2X) and then opens up just before front foot strike (3X). All in all, don't "squish the bug" in pitching. Get everything going toward the catcher and you will be fine.