Are you setting specific goals? I went from a 12 to a 1 in about 5 years. A big part of that progress was setting goals at the beginning of each season. I would set 5 goals at the beginning of each season. Set goals that you have control over. For example, these are just a couple goals that helped me:
1) Play in 5 competitive tournaments
2) Practice putting for 1 hour/week and track progress during rounds
I like to spend the winter brainstorming and thinking about my goals and then I write them down and keep them handy so I can refer to them and check in with myself during the season.
Also, I've talked with others about "breakthrough rounds" and these can really accelerate your progress. Unfortunately, you don't know when it will happen, but it will build your confidence when it does. For example, the first time I broke 80 I shot a 75. Going so much lower than I was accustomed to helped me realize that it wasn't a big deal, and then I started shooting in the 70s more often. You really have to let your great rounds build your confidence...too many golfers make the mistake of dismissing these rounds as flukes.