Tennis - improve methods

I have been playing tennis for over 15 years and at first I am just like what you said. But first of all, you should make sure that your form is alright.

Then you should check whether you hit the ball at the 'centre' of the racket or not. Hitting at the centre of the racket is of course more powerful that at the edge of the racket.

Then you should make sure you do the "follow through" swing with power of your waist.

Apart from the above, to my experience, it is always that you are not well prepared for the shot before you hit the ball, so that your swing is not complete. This is due to that you have to foresee the coming of the ball and predict its bouncing direction, whether it is a sliced shot, a top spin shot, power shot with no spin, etc. But this will depends on the experience. So when you play next time, try to predict the tracking of the ball so that you have enough time for the preparation of swing for the shot and this will undoubtedly increase the accuracy and power of your shot.

I have been playing tennis for over 10 years and my experience tells me that most of the time it is your form. You can not tell how good your form is. You need someone to look at your form and give you comments, or you can video yourself and check later on.
Power is not generated by your arms, but your waist (or torque). In other words, you need to turn enough when you prepare to hit and follow through every single hit (both forehand and backhand).

Hope this helps..