Well, for me, we have a four year old, with joint legal custody, but I have primary physical custody. Our arrangement is he has our son every other weekend from friday afternoon until sunday night, and then every wednesday night. North Carolina determines custody TIME by the number of overnights each parent has, so it works out that he technically has him something like 56 nights a year. So, in effect, I don't only have my son half the time...I have him quite a bit more. Now, my EX and I get a long well and we help each other out, but please don't think you will only see them HALF the time because, with primary custody, you will see them much more.
I understand the feelings of disappointment, etc you feel...as if you're "letting your kids down." Here's how I look at it: I would be letting my son down more by staying in a strained, tense, unhappy marriage versus being divorced, but knowing we are both capable of giving lovingly to our son. Yes, it's not ideal. In a perfect world, I wish we could be happily married and get on with life like my parents who will be celebrating their 40th this year. But, you have to do for YOU, and a better you is a better Mom for your kids.
If you two can work effectively as coparents, really have an open line of communication where the kids are concerned, then your children will blossom.
Sometimes we need to make sacrifices for our kids, but that's what parents do. Wishing you well! - Trying.

If he's abusive I would petition the courts for full custody. You're exactly right in feeling that once you're divoced you wont be there to protect your kids when he becomes enraged and stressed out. There will be NO one to protect them. You need to protect them NOW so that that will never happen. I have/am going through the same thing so I know what you're going through.

I am probably biased, but - I suggest counseling - both individual and marital. Secondly, if you think you might be co-dependent, then you may just be oversensitive to his behavior - just a thought. So, you may want to try al-anon, or some other 12 step group that may apply to what you are dealing with. Divorce is not a decision to be taken lightly. If he is physically abusive, then get out now. If he is verbally abusive, then get to marriage counseling. If necessary, give an ultimatum. But - understand, you would be taking your kids from yourself, and their father for long periods of time. It will have a profound affect on your children. So - in closing I say; get help for your marriage, and if he is unwilling to change or to go to counseling, then get help for yourself. Then - make a decision. I am biased because my wife is seeking divorce for a second time. She is severely co-dependent and is only now realizing it. If this goes through, I don't know if I will be able to take her back again. I don't think the kids will quite forgive her either. I know that she doesn't forgive her mom for doing the same thing.
Hope this helps