Don't worry, you're not a horrible person or terrible mom. Experience is a great teacher, so just move forward from here.
What does your husband think of the boys' situation? Is he home enough to handle their fighting? If he is, have a family meeting and explain that their dad will be the one in charge of any arguing or hitting, and he'll decide what happens. If it happens when he's not home, inmmediately separate them, but hold any consequence until Dad is there to handle it.
If your husband has always either encouraged or ignored the behavior, find a family or couples counselor to help everyone figure out healthier ways to behave. Their dad is probably either baffled or thinks this is "normal" behavior, and really isn't sure what to do.
Even if your husband is dealing with the fights, you are still a parent, and you can make rules. Try to prevent the fights from happening by keeping the boys away from each other. Tell the boys that they'll be separated until they can get along. (Make it clear to the victimized brother that you're protecting, not punishing, him.) Keep the boys in separate rooms while they work, play and sleep, even if it means switching bedrooms. When they must be in the same room, like at the dinner table, seat them at opposite ends and make it clear that they can only speak to each other if it's polite ("Good job on that report,") or necessary ("Please pass the potatoes.")
Watch and talk to the victimized brother to see what sets off the fighting. Maybe he is either deliberately or unconsiously making his brother angry by doing something he knows will upset his brother. He needs to analyze how his behavior - ignoring, gloating, complaining, not asking to borrow things, or anything else - contributes to the attacks, and he needs to know how and why to change his role in things.
Find a family or childhood counselor. Things won't really get better until the agressor knows why he's doing this, and what he should do instead. Is he jealous - think his brother is loved more, fussed over more, has more friends, better privileges, better grades, is more well liked by teachers and other kids? Or, does any real or perceived weakness make him want to take advantage of the "weaker" person? Why? Or, is this just a habit left from toddler years and he just doesn't know how, or why, to stop?
Good luck, Mom! Hang in there!