Some people develop a lisp under stress or when they are tired or allergic.
Try recording your speaking voice with the microphone about 8 to 12 inches away from your mouth. Sit up straight, and read something aloud for at least 3 or 4 minutes or until you are no longer self-conscious. After reading aloud, then sing a few very familiar stanzas without any regard for intonation or anything else (again, so that you stay relaxed). Listen to the playback -- is the lisp still there?
Then ask someone you trust to be honest with you whether you lisp when you speak, and if so, is it all the time or under certain circumstances? Ask the person to listen to the recording, too.
Often, because of bone conduction through the head, people think they sound one way and find even accurate recordings of their own voices repulsive when everyone else thinks they sound just fine.
The more experience you gain with hearing your own recorded voice, the less difficulty you will have with the way it sounds, and the more you can modify it to your liking. Just as you can develop your singing voice, so too can you develop your speaking voice. Recording and listening to playback of your voice (without judging yourself negatively for what you hear) are essential to voice training, in my opinion.
Stay with it. You'll definitely improve.
Best wishes,
Mark H.