After I read this, I called my mom and husband at work to get their input.
My husband and little brother went through speech therapy in grade school; my brother only for a year, but my husband didn't get help until 5th grade. My brother had trouble pronouncing a few letters and would stutter, but it was taken care of by the end of 1st grade. My husband, however, still has a slight lisp when he talks too fast or is excited (usually involving a new computer gadget) which I think is cute, but then again, he's my husband. :P
I know speech problems are frustrating, but give it some time, things will work out. Having patience is key, my husband says. He said my in-laws never pushed him or corrected him, just let him work it out on his own. As time went on, he would catch himself more and would learn what was triggering his slurring/lisping. As for my brother, Mom says he just seemed to outgrow it.
My husband said if you'd like to talk to him about the therapy he went through in school and what my in-laws did to help, let me know. He'd be more than happy to give you his email address if you're looking for more information. Hang in there, it will get better! :)