I had a speech imp-imped-ah... a stutter - I went through speech training and later vocal training. The last is what actors go through to learn the nuances of accents, thus the guy who plays Wolverine sounds like an American but has one of those cute Oz accents in real life....
I still stutter under high emotional states. I have to stand/sit there and consciously put effort in slowing down my head and my mouth. There is much to be said for counting to ten slowly in these situations.
The downside to that is people want to help and keep from finishing sentences for you. That can frustrate you even more.
Know someone who stutters, then don't ever help them find their word(s).
Speech pathology is based on several factors, including (but not limited to) brain disorders, nerve disorders and malformation of the speech equipment: mouth (teeth, set of jaw), tongue and even vocal cords.
Children who loose their front teeth will take on a hissing sound because the way the tip of the tongue hits the front teeth to form s's. If you have a gap between your front teeth, or if they are crooked it is possible that that affects your use of S.
So if there is a physical issue, get that addressed first. Assuming there is noting wrong with teeth, mouth, tongue then you need to start speaking slower and enunciate.
One can retrain one's own voice. "She Sells Seashells on the sea shore" is one way to train your s's. Tongue twisters do serve real purpose.
If you have prolonged s's then you need to start consciously cutting them off. If yur s's shound more like 'th' so instead of saw you say 'thaw' or 'taw' then you need tounge retraining. You need to consciously think about how you are using that tongue.
Assuming you didn't have the issue before hand, and when you came out you started hanging around lisping gays and picked up the habit... well its a habit. One can break a habit in pretty much the same way they got the habit - by doing the 'right' or 'other' thing.
I note that a lot of gay guys do lisp more when they hang with other lispers. So it is in those situations you most likely are going to have to force yourself to be more conscious about how you are speaking.