I've read some good comments in here. Lito for one. It is a fundamental rule of any athlete that you CAN improve on performance. I have a grand daughter who runs track. When she started she was fast. But over the past two years she has trained under a good coach and she got FASTER, and placed fourth at state level. This year, she will most likely get a scholarship for college because she has even improved on her past record. If it were true that you can not improve beyond what you are, there would be no hope for anyone in sports.
Just as a addition to the comments made I would recommend you go back to the basics Get on the heavy bag and start with snapping your punches quckly and returning to your defensive stance. Dont worry at first about how hard you are hitting. Keep your stance and your balance, then get into moving with your snap punching. Do not bulk up pumping iron. Have someone with a stop watch time your combinaton punches...that will give you a base line to work on speed. Practicing snap punches will improve your speed. It's a prove fact. Then you start putting some power behind those punches with your body and you are good to go. It's hard to type really so many other thngs that might factor in impovement. Hope this gives you some insight. I am 74 years old and fought with and trained hundreds of boxers, and beyond the quitters and bums, the fighter who listened to his trainer, himself, and others more skilled, I never saw anyone who did not improve.