Is my severe overbite causing my speech impediment?

Yes: Part of an orthodontic screening is to evaluate function, including speech. Often speech irregularities and malocclusions go hand in hand. Ask a qualified orthodontist to examine you and, if necessary, as for consultation with a speech pathologist.

Definitely, yes: Once an overbite has developed, it can cause several physical ailments like speech impediments, such as lisps.