It sounds like you are using pasteurized homogenized milk. Raw milk is much better, but if you can't get that or just don't want to spend the extra while you learn, here are some tricks that have worked for me.
1. Go to the hot side of the range for mesophilic starter
2. Make your initial curd cuts large, about 1" apart in a cross hatch, and let them settle for 10 minutes. Don't make your horizontal cuts yet.
3. I use a bent wire to make my horizontal cuts. Make them as gently as you can.
4. At first, just jiggle the curd, and work very gently. I use the narrow handle of my ladle for this. As the curd starts to shrink, try to gently move the bottom curds to the top.
5. As the curd shrinks it will firm up. Cut any large pieces with a knife. The curd will enevitably break up some; that's why I start with such a large cut. As the curds cook you can stir them a bit less gently. Cut any large curds that haven't broken. You want a consistent curd size above all.
6. Don't worry about the small broken pieces; they will knit back together.
P/H milk doesn't have the flavor of raw milk, so I add my starter culture to the milk the night before and keep it in the refrigerator. This does improve the flavor.
May I suggest that Caerphilly is a great cheese to learn on? It can be eaten as young as 3 weeks so it speeds up the learning curve. Take notes on each make, so that you can see the effects in the final cheese.
When I first joined this forum, I read through all of the archives in the Problems board. I learned a great deal and avoided a lot of errors.
Dave in CT (that rainy gray spot in the north east United States)