Please help me stop lisping?

I had a lisp when I was little until I was seven. I couldn't pronounce the TH sound. My dentist put in a tongue thrust device and it stopped it almost immediately. Ever since I got my braces I had trouble pronouncing the S sound, and every one said it would go away. I had my braces taken off 7 months ago and now my lisp is worse! I am in high school and my friends think it is cute and funny, but it is really humiliating. I would lisp once in a while before, but now it is all the time. A few months ago my parents and sister agreed that I do have a lisp, but now they say I don't. I work at the elementary school, and the kids haven't commented on my lisp, but I don't want them and my future students when I become a teacher to have to deal with that. I want to teach English. It would look really dumb if I was teaching a language I can't even speak correctly. This is really killing my self esteem, and it's making me re-evaluate my friendships because they make fun of it. I really don't have time for speech therapy classes. Is there any kind of technique I can do at home, or is there anything my dentist/orthodontist can do? I used to suck my thumb when I was little which made me lisp on my TH sounds, could it be causing the S lisps too?