Do you know how i could improve my speech?

i have a bit of a speech impediment and it causes me to be extremely shy. (to the point that if somebody speaks to me I'll pretend I didn't hear them or just walk away because I used to be made fun of a lot in elementary school) i wish to be more comfortable with myself and trusting of others because it has caused me to be very suspicious of other people, always thinking that they're mocking my voice behind my back. I have a bit of a lisp and I friggin hate it. Is there anything I can do to help myself. i have no desire to see a specialist about this because i seen them as a child and they did nothing. Thanks.