Can you practice writing dialogue in the voices of people you know well? Fer instance, in my life, there are lots of unique voices:
-I read too many books and use lots of weird phrases that people don't often use. i have one friend who speaks the same way. sometimes people tell us it rubs off on them.
-My partner at all times bounces between west coast black urban accent, a vicious brummie accent (you know, from Birmingham (UK)), and the RP (you know, fancy brit english like jude law). strange but true.
-a friend who moved from Switzerland to the US in first grade uses lots of unusual grammatical constructions, speaks very carefully, is a super genius, and ends nearly every story by dissolving into a fit of schoolboy giggles.
-my landlord is an armenian immigrant with a thick accent and almost-but-not-quite correct grammar
-people from texas, people from jamaica, people with autism, fast-talking salesmen, laid-back hippie surfers from monterey, people who are trying to impress you with their cleverness, people who are manic, people who have lost all hope, drunk people...everyone says things differently. practice with some styles of speaking you are already familiar with, rather than try to invent whole new ones at first. Your characters can 'borrow' the speaking styles of people you know, and adapt them to suit their personalities as well.