Sairys, it sounds like you're doing everything right and have great opportunities at your barn. Definitely get some lunge lessons, you're exactly right about their benefits. They really help you develop your seat and a feel for the horse, and they're not just for beginners. Everybody should go back to them from time to time.
When you do your practice rides, give yourself a goal of something you want to work on each time. Ask your instructor to give you "homework" for that. Otherwise you might find you're not focusing enough. Also, if you find you're having a problem during those rides, don't hesitate to ask your instructor for help, or even some other more advanced student.
By the way, another thing that can really help you is to have someone videotape you riding. But keep doing what you're doing, it sounds like you're on the right track. And save up those quarters for your very own horsey. I was over 50 when I got my first horse, and it was such a revelation to me. I had to sell her when I moved, and I can't wait to get another one.