Oh my, this is a problem. I've had it as well. Very tricky indeed.
How do I get outside myself? How do I think and talk like someone else?
How can I stop being me??
You know, it occurs to me that actors have this problem as well. Some of them take their jobs really seriously and get deep into character and pretend to really be that person. They change their bodies, their lifestyles, their personalities. They want terribly to convince you that they are no longer the person you know they really are.
I've never heard of an author doing this. I wonder why?
Perhaps because we have the advantage of not needing to be that person on the spot. We can think it out, take our time, revise, edit, sculpt, until we've created the perfect portrayal. We have all the time in the world. They have to do it all in one take, or start over.
So my first suggestion is to take massive advantage of your ability to fine-tune every frame, so to speak. Look at every sentence, every word, every phrase and expression. Look at the overall structure/direction of what the character has said in their writing. Is it consistent with their personality? Would Abraham Lincoln really use the word "bubblicious"? Of course he would.
My second suggestion is to simply get into your character. Think about them. Sketch them out. Figure out their past. Imagine their future. Mentally place them in a variety of situations, talking with a variety of people, and discover what they say, and how they say it. I think you might find an image taking shape very quickly. Once you really know them, you'll know what they would say without thinking twice.
The proof's in the pudding. Take your close friends, people you know better than anyone. If one of them did something out of character, wouldn't you notice instantly? You'd think, That's strange, James is never this direct, or How unusual, Sarah using a word like "effluvium"? When you really know someone, breaches of standard behavior stand out like neon lights.
And don't think that characters are any different from close friends. After all, who knows your characters better than you, their creator?