Don't start by saying you can't. First you have to learn how. If you don't learn how, you can't. If you don't try, you can't either. But aside from that, I can't (that word again!) think of any reason you can't improve.
Don't expect to become an overnight success, though. When you learn something for many years, you can't unlearn it - AND substitute something better in its stead - in a day, a week, or even a year. But you can make a start in less than a day.
You've made a HUGE step already - you have recognized out loud, so to speak, that there is a problem. What do you think the next good step might be? A visit with a counselor? A book or two to read? A course in tackling negative behavior? (If that last one is something to consider, I recommend Zig Ziglar's "Born to Win" course, on CD/DVD. It's not cheap, but it's worth every penny.)
Here are some things you can do to take some baby steps in the right direction. Think about all your friends; is there one whom you admire for her positive attitude and her communication skills with her children? Talk to her, and ask her to teach you how *she* does things. Ask if you might pick her brains. You can learn a lot from a good friend.
If you have some sort of device at home that can record your voice, turn it on when you're at home. Carry it around with you. This will make you aware of how you sound to others. Find out what you sound like in general, *and* how you sound when you raise your voice. Once you hear yourself, maybe you can set a goal that, when you feel like getting louder, you get softer instead. You're going to feel weird - and the people around you are going to wonder if you're weird, too - but don't let that stop you. When your emotions go up, your volume goes down. Trust me. It will help.
I am giving you a standing ovation right now (even though you can't see it) for setting this goal for yourself. Go for it, and don't quit!