You should seriously get rid of the mirror in general, especially since your goal is to compete. You will not have access to mirrors in competition, and training without one and competing without one is going to screw you up.
The squatting with a mirror thing is a pretty common phenomenon. You become reliant on visual cues rather than obtaining body awareness. In all honesty, you shouldn't be concerning yourself with form: you should be concerned with technique. How the movement looks is completely immaterial to your success, it's what you're doing during the movement that matters.
Like, you could look at yourself deadlifting and see you have a rounded back. Great. All you've done is identify "bad form". HOWEVER, if you realize that your feet are too far apart and it's preventing you from getting your hips under your shoulders at the start of the pull which is forcing you to stiff leg the initial pull and causing your back to round, you've now identified improper technique. A mirror does nothing to help you identify the latter, that's simply a question of body awareness.
If you're really itching to see how you look while you're lifting, record your sessions.