Trills are tricky, lizz. Many folks get a mental block over them. But once you get the trick, they're a total fun-zone.
First off, don't bear down with tension when you approach them. RELAX. Totally. Let your hand/arm drop onto the keyboard and rest your hand there, with the two fingers on the keys you will be trilling with. Say, 2 & 3 on C and D with the right hand. Now, very slowly, play 3 (D) and feel how the weight of your hand flops over, ever so slightly, to the right. Actually exaggerate this feeling with a tilt of the hand to the right. Now, next, let your hand transfer its weight to 2 (C) and feel the weight shift to the left. Repeat. It's like a rocking movement, yes? You see, trills are not just the quick oscillation of fingers, but the engagement of the entire hand and the arm to which it is attached. The fingers, actually are just the guidance system.
Naturally, as you speed up the playing of the two notes, the rocking motion gets less and less perceptible to the eye, but it's still there and if you are not totally relaxed, you won't be able to get the speed up. And if you DO get the speed up with a stiff mechanism, you'll experience tension and some pain. Ultimately, the trill will be unplayable.
Practice this way, very slowly, and concentrate on RELAXATION. You'll get it in time.
Hope this helps.