Well, if you lack confidence in your strokes, why did you ask about your vision? Also, the "juggling" that Gold Chick talks about is not juggling - it's just hitting several ball quickly that are fed to you. Geepers! Ok, sorry, just had to get that out of my system.
To answer your FIRST question, (and contrary to what the others said), I'd say you NEED to concentrate on the ball primarily, and watch your opponents position with your peripheral vision. The ball is small and moving quickly, and you need to hit it - it deserves your focus. Your opponent is big and (relatively) slow - and you DON'T have to hit him - he does NOT deserve your focus. You can usually see your opponent with your peripheral vision. If you can't, don't worry about it - just hit your shot. If you happen to hit it at him, no big deal. Just concentrate on the ball and give it a rip!
To answer your SECOND question, nothing builds confidence in a stroke like good mechanics and LOTS of repition. Build good mechanics by working with a pro, or getting advice from either the internet or some good books. Then, hit the court and groove those strokes so you can hit them over and over. You can work with a wall, a ball machine, or a practice partner. You need good strokes, and you need to have CONFIDENCE in your strokes.