The other thing in learning to trim is to understand is what you're trimming for.
At this point, you are learning to trim for straight and level flight, which is good. As said, best check is to let go of the stick/yoke altogether and see if the aircraft maintains the proper attitude. If it doesn't, don't try to correct the trim straight away but first use the stick/yoke to regain the proper attitude, then adjust the trim again and try again.
As others have said: a properly trimmed aircraft can be flown (and is flown, actually) hands-off, where you use rudder to initiate or check a small turn. This leaves both hands, and your lap, free for charts, rulers and other navigation stuff.
But in the course of your training, eventually you may or may not have to or want to trim for other attitudes as well. For instance, are you going to trim the aircraft for a long climb (from take-off to 3500' - which is a long climb in a 152), or for a long descent, or for other attitudes? Are you going to re-trim the aircraft on downwind, and again on final?
To answer these questions, go back to the beginning. The reason for trimming an aircraft is to reduce or eliminate permanent control forces. Either with the objective to fly the aircraft hands-off, or with the objective of making your controls more sensitive. So I tend to trim the aircraft during a climb, during straight and level flight, and again for the descend & downwind. I normally do not retrim on final, unless it's some kind of precision/short field landing and I want the sensitivity in the stick. In fact, even on short final, when doing a precision landing, I try to nail the speed first, then trim the control forces out and then quickly let go to see if the trim is correct. I then fly the aircraft onto the runway with throttle alone, with maybe just a tiny bit of pressure from my thumb or index finger. But that pressure is more to anticipate the effect that an altered throttle setting has on the attitude than anything else. (Until the flare of course.)
But I'm also doing an aerobatics course right now. Before starting the aeros we trim for a fast cruise, but then leave the trim wheel alone. Slow flight, stalls, spins, inverted flight, but also 4g loops are all done by heaving on the stick without retrimming. Because the amount of force on the stick tells you to what extent you are forcing the aircraft into doing something it doesn't naturally want to do.
So eventually you're going to have to work out for yourself when to retrim and when not to. Basically, you retrim for any attitude that you will want to keep for a longer period of time, and you don't retrim for short, transient manoeuvres.