Yes I understand this is definitely frustrating and can get you down, I would say you’re doing the right things though, working on the placement of your shots, really try to find the opponents body more often, this makes it much harder to play a flat hard shot as they have to adjust the position a lot.
I would also say its not all about the speed of your shots, it’s important to play with a lot of spin and this again makes it hard to just wack it back like you said. So really focus on improving the contact and spin in your strokes. Using the body and legs is key to generate spin and power, and the wrist to accelerate through the contact and get more spin.
Lastly I would say not matter what age you are you can still improve, and in the long term you will overtake these players who are just wacking it with flat shots, so don’t be discouraged, keep working on the right shots and it will pay off!
