I think I can probably help...
By this I'm assuming you're talking about force plays (or the equivalent at first for the pedants among us). If it's tag plays that are giving you fits, let us know and there will probably be different advice.
1) It's easy (especially when you're new) to try to get too close to a force play. Your eyes can only see so wide at once - think of it like having a dinner plate at arms length. That's the part you can *really* see. Try backing up and watching from farther away.
2) Make a concerted effort to watch the foot on the base. Keep watching it until *after* the runner touches the base. When you're in A, if you look up too soon, you can make a snap judgment. When you're in B or C (and this is something I *still* catch myself doing), you find yourself looking at the glove instead of the base. It's one of the few times you can actually see the ball when it hits the glove ... but you shouldn't, because you should still be watching the base and listening for the glove
3) And this is the big one. You're still rushing yourself. Your brain is going 1000 mph and you feel an internal pressure to make a call right away. I was probably 5 years in before I got the feeling "oh, this is what good timing feels like". Watch the play. Keep watching the play. Re-play it in your head. Wait for the fielder to do something else (throw the ball elsewhere, run toward the pitcher or off the field, look up toward you to see what your call is). THEN make your call.
4) Related to (3). When the ball is on the way to the fielder, exhale and leave your breath out until you make your call. This does two things. (a) It slows your nervous system down and lets you concentrate, and (b) it forces you to inhale before you actually make your call, slowing you down even further.
As said earlier, don't beat yourself up too much. It happens to all of us. And as the first guy who got me interested in umpiring said "it's the only job you'll ever have where you're expected to start perfect and then get better every time"