Whilst there are many reasons why you may play like this I believe there is one thing you can do to improve consistency, and that is, to improve your awareness skills. The more you're aware of what you're doing when preparing to play, the more you'll be able to control. I coach many golfers who unknowingly tighten this or that muscle during their preparation. They might do it for one shot, and then not for the next. This brings an inconsistency to their preparation and therefore to the shot. The important things here is that they're unaware they're doing it. If they don't know they're doing it they can't control it!
Using simple techniques such as being aware of your breathing or the ground under your feet can help to get you into a quiet state of mind to notice more of what you're doing. When you can do this you'll have more control over your actions and give you a better chance of playing the shot how you really want to.
See how you can apply this teon the course at a simple golf swing tip.