You think we have no problem with it? Finding the individual voice of a character is, quite possibly, the most difficult thing you will ever do as a writer.
You need to read more. I know it's hard- and as you get busier with the "real world" it gets even more difficult. But make the time. Read on your lunch breaks. Get audio books (I know they come on disk or downloadable for your phone/mp3) and listen to them in the car, or while mowing or cleaning or doing the dishes.
Invest in a both a dictionary and a thesaurus. These are the most crucial tools you can have as a writer. Keep them beside you as you write.
Find someone you trust, someone with either experience as a writer or a good grasp of the dynamics involved, and ask them for honest feedback on what you've written. (More important still? Get a thick skin because not all critiques and criticism is comfortable!)
If you don't like your writing style break it down. Find where the flaw is. Have someone help you find that flaw if needed. If you must, change your style- study several different writing styles and find what works for you.
When you've found a flaw? Write in a way to specifically target that flaw. Keep writing. You may throw away a hundred pages for every one you keep, but keep at it!
And always remember- a writer's worst critic is him(her)self. What you spot as glaring issues may be barely noticeable to someone else.
When you can't run, you crawl. And when you can't crawl, you find someone to carry you.
We have done the impossible, and that makes us mighty.