Have you tried straight arm work? Maximal contraction and strength in a straight arm position is a core aspect of a powerful punch. It's the difference of being hit by a sack of sand or dirt, and hit by an iron bar. Besides that is the weak muscles in your hips, low back, glutes, feet, ankles, calves, knees... etc. The kinetic chain. The transfer of power, torque, tension... can be painful. Finding pain when trying to hit harder is a good thing. Or slowness, if your body stops you before hurting yourself. If you apply a "battle to the death" mentality though, you will push the weak and injury-prone tissue into action rather than your body stopping yourself. This is a good thing, as you know your weak points now. You can take it safe and go for some strength and endurance training to shore up the weakness (straight/long limb being best usually, as your joints an dbody contract maximally in order to stay together and avoid injury aka strengthen). That's why active stretching can be very good; such as getting into a split and holding your body above the injury point with your own muscle. Similar principle, just make sure you can hold yourself up and have something to catch yourself with.
Connective tissues around your joints have to be incredible strong in order to decrease the space that the liquid in your joints is in whilst having the strength to include dynamism in the liquid's pressure changes in order to act effectively as shocks. Weak joint muscle/connective tissues can't squeeze in a controlled and appropriate manner to distribute force safely by manipulating the liquid and bones methodically and vigorously.
You might not have a problem with power now, but I hope my advice is something new and you can recognize the sense in it.
I work out and stretch almost every day - with a variety of exercise, including core, balancing, strength, and cardio. I don't think you fully understand the kind of physical limitations I am talking about. I'll give you an example: I have osteoma in both hips. It is a benign bone growth, not anything lethal, but something that can seriously limit how far your joint can go and in which direction. In my case, to execute a high round kick parallel to the floor in the form we were taught as classical, I have to lean back so far I can no longer see my target. Otherwise, I literally cannot get my leg up - my joint doesn't work that way. So, I don't do those particular kicks. I had to develop a modification, where I can reach both height and strength in a different way.
I don't think it's ever a good idea to absolutely state that "pain is a good thing". There is soreness from exercise - it happens. That type of pain eventually goes away as muscles develop. However, there is also pain from tearing tissue or a cracked bone. Telling someone it's a good thing, because it's all part of the practice is misleading, cruel, and downright harmful, because it can worsen the injury and impair one's ability to train for the rest of his or her life.
