I have a lisp because of my tmj splint. how do i get rid of this?

Tell the dentist: You need to inform the dentist who made the splint. If it was bought over-the-counter or online, then i would see a dentist anyways because those types of mouthpieces (non-custom) can create some permanent problems- moving teeth, etc.

Upper or lower: If you have an upper appliance it is quite common to have it affect your speech. You may consider using a lower appliance as it does not affect your speech in the same way, but you might also need the upper appliance as a night time appliance.

Adjustment: It's natural to have speech changes with anything put into your mouth. Usually a simple adjustment can take care of speech issues lasting for a while. Usually your mouth and tongue will learn the appliance and will be back to normal shortly.

Ask Dentist: Ask dentist. He may be able to modify splint or switch to another type.

Be patient: Once your mouth gets used to having the TMJ splint, you will go back to talking normal, again in a few weeks.