I know exactly what you mean.
I had an expander for a few months, and I couldn't say any words that had an "E" or "I" in them. I never had a lisp with the expander... boy am I happy that I don't have it anymore. When I got braces, I also got turbos. I had a lisp for about 5 minutes, then it went away

. Just try not to be self conscious about it, and speak a little bit slower than usual.
Also, thesaurisize the words you can't pronounce so you can avoid them
Don't worry, pretty soon you'll be able to pronounce almost anything.
Oh, and Google the Magic Passage (I think that's what it's called) and recite it a few times. It helps!