If you have the muscles already developed, then you just need to continue to consciously make an effort to activate them when they should be activated. It sounds like you are also in the third phase of the example that I cited on Dancing and the core. You just need to keep it up so that it becomes second nature to you as well.
I must confess, as a programmer also, I sit in a chair a large portion of my day, and during that time in my chair, I slump a lot. I'm working to correct this, and so every time I realize that I'm doing it, I make a conscious effort to correct it. I've also switched to sitting on an exercise ball the last half of the day as well. It doesn't "make you sit straight" but the muscles in your back start to bother you a lot quicker if you assume a poor posture which reminds you to sit up straight quicker.
I will agree that self-esteem will play a large part in your posture as well, and would suggest that if you have any serious self-esteem issues that you see a counselor. Low self-worth leads to a lot of other complications as well, including motivation and health. Leading a healthy lifestyle of diet and nutrition and feeling good about oneself is very important. I'd say that if you've been doing kung fu and tai chi for 3.5 years, you should be quite proud of your accomplishments to date.