1) You can improve your serve at home in your garden with some foam balls. To get a rhythm on my serve I just grab my racquet and serve in my garden or anywhere that's convenient. This way I can really go to town and explore what works for me. I can also slow my technique down and see what's getting in the way while still doing a proper serve. My dog loves fetching the ball for me.
2) Have you heard of the all court forcing game
. I have the same problem against certain opponents. What I have found is that my game essentially feeds them. I give them the shots they like. To beat them I have to go outside my comfort zone and often not attack as much.
It seemed weird at first. Rallying more and waiting but I learnt it was about court positioning and waiting for the right ball. Fuzzyyellowballs
did some online strategy courses which really helped me with this. Understanding which shots hurt your opponent and which might actually hurt you. The inside out forehand can be a good example.
What I basically found is that the harder I attacked the faster the ball would come back. I made all the effort and my opponents just redirected it back at me. So I would often still be landing from my serve when it flew by me out of reach. So I learnt how to vary my spins to vary the pace of the ball. I often use slice to slow the ball up so I can get in good position so my opponent has a smaller target to aim at for winners.
Let me know if this is any use to you. It sounds more like a strategy issue than a technical one. Fitness is very helpful but it is most important to know when and how to use your shots instead of just hitting what feels good.
There is a lot of good online instruction which I use instead of paying a coach. Essentialtennis, Jim Mclennan and FuzzyYellowBalls are a couple of examples.