They all sound like me!

Dialogue in novels are not exactly the same as real life dialogue. IMHO, the best way is to study many novels to see how other writers do it.

 

i'll ditto that, bruce!... as long as the novels you study are by good writers, that is!

 

I've never had this particular problem. Sometimes when I am stumped at something to do with my characters, I get the unusual and weird sensation that the characters that I envision in my mind wouldn't be in my mind if it weren't for me. You created your characters. You know them better than anyone. I suggest some writing. Use these characters and write scenes that you may not use in your novel. Just write and teach yourself and learn more about them.
You say they all sound like you?

That's not very surprising. In a way, we are all full of stories. I learned something along the lines of this in both my psychology class and my narrative class. We can be thought of as just groups of characters. Maybe part of me that wishes he was the lady's man is in fact the lady's man in my novel. My point is, you create them, you basically are them, so your going to sound like them. The trick is to disguise this familiarity between yourself and your characters. Mannerisms can help too. Accents, slang, etc.

Of course, my opinion, as usual.