Do NOT try to serve [1st or 2nd] serves at 135 mph. 95-100 mph is plenty. It is critical that you get your first serve IN at least 65-70% of the time.
When you get your first serve, it puts pressure on the receiver. Miss the first serve, and the pressure is on the server. Underhand serves can have an especially obnoxious level of side-spin if you slice past the ball just right.
Serving is by far the hardest stroke in the game, but as you know, you can work on it on your own. There are millions of other tennis players who share your frustration with it. It takes at least a few years to become a strong player and serving is a big part of that.
Get used to serving at about 90-95% of your top speed, whatever that might be. You’re still attacking the ball, just not wildly as you would if you were trying to set a new world speed record. The payoff will be when you can use your 95% serve confidently as a SECOND serve, because most players aren’t used to seeing a strong second serve at the high school level. I hope you learn a good kick serve and/or a slice serve to give you some variety. Sometimes a slow ball on the first serve can surprise a guy, mess up his timing, and give you one or two cheap points. Don’t do it too often, of course. A common question we get is What are some effective ways to perfect your serving in tennis
I used to have a “stand still” serve that I used anytime I was in trouble and absolutely HAD to get it in. Same basic motion, but no fancy footwork down below. You can still hit a very strong serve with both feet on the ground. Try it sometime. The heel of the back foot can go up as you rotate into the hit, but the toe is still “grounded.”
Don’t be too hard on yourself. Stay with it, keep practicing, asking your coach questions, etc, and you WILL get better. It doesn’t happen overnight.