You can learn a lot from your mistakes if you accept all of them. If you are not getting better it is probably because you are not preparing yourself to go onto the golf course and succeed. To change this, find a better, more time effective, way to practice.
I suggest more rehearsal swings and less hitting if you are making a change or trying to fix an issue. Don't get me wrong you need to hit a lot of balls to make a change permanent, but not without rehearsing the new motion. There is a huge difference between "rehearsal" and "practice" swings!
Practice swings are for tempo in the swing and warming up. Rehearsal swings should have more thought. In a rehearsal swing you should be trying to feel what is in effort to be changed. When making a rehearsal swing I am trying to feel a position in the sequence of the motion I am making. A rehearsal swing should always be slower so that you can do what you are trying to do. The slower pace of the swing allows your brain a better chance to isolate the part of the body that is making the adjustment in the swing. Remember if you cannot feel what you are trying to do, you cannot do it. Make rehearsal swings so you can feel the change and get better. Sometimes all it takes is a new feeling to get the confidence going in a more positive direction.
Lastly, do not fear the failure of change. You are going to fail way more times than you will ever succeed. Even without making a change every golfer hits bad shots. The greatest athletes of all time fail much more than they succeed. Elite basketball players miss way more game-winners than they hit. We, as fans, remember the game winners that go in a lot more than the ones that hit rim and do not go in. Good baseball players hit .300. So the best baseball players typically fail 70 percent of the time. Accepting change gets easier if you accept all of your mistakes and learn from them. Look at Rory McIlroy: He blew a huge lead in the last round at the 2011 Masters and he came back at the next major tournament, the 2011 US Open, and won easily. Mentally, Rory had appeared to become better than ever. Why? Because he accepted all of his mistakes, learned from his mistakes, and fixed the mental and physical issues. You learn from them, you get over it, and you come back even stronger if you make the appropriate changes.
If you are happy with the current state of your golf game, practice to sustain your abilities, and have fun on the course. If you are not happy with the state of your golf game, then you must figure out a new way to do things. Contact a local PGA Professional for a golf lesson to figure out where you are going wrong and fix the problem. Remember, Change is good -- especially if you are unhappy with your current results.