The lisping might be partly because of malocclusion of your teeth and you might also be having a tongue thrusting habit wherein while pronouncing certain words or letters(especially with 'S's) , your tongue might be pressing incorrectly against your front teeth, a childhood pattern which usually corrects with age in most but some people. In any case, get an orthodontic consultation done and then only you'll get a correct diagnosis after a clinical check up. If this is the scenario, a simple tongue crib appliance therapy and braces might help you resolve the issue. All the best.
Next Steps
Consult an orthodontist
Health Tips
Some people may also need additional speech therapy. Also consult a specialist orthodontist. The treatment should last 14-18 months depending upon various factors.