Sounds to me like you're seriously confused. In your first paragraph you say you're not sure if you want your marriage to work and in your second paragraph you say you really want a divorce.
The whole cheating bit only clouds your vision and zaps the energy you could be using to improve your marriage. I think more of the issue is whether you have the spine to step up to the plate and delve into the issues of your marriage, or if you only want to keep everything superficial and continue cheating for the cheap thrill and lack of confrontation. The cheating is the easy way out. No one needs to know, no one gets hurt, you don't have to put any energy into it, other than covering it up. To tell your husband you want a divorce makes you look like the bad guy, it opens up pain and anguish that would have been avoided had you just kept your mouth shut.
If you're so worried about hurting your husband, you do love him then. And if you're sure he'll be so hurt, then he loves you as well. And that is the spark that can reignite the marriage, but only if you put your energy into that and not into the other man.
12 years is a time frame when things start to get a little dull in the relationship and you'll start thinking about all the bad aspects of your life rather than the good. The grass ain't always greener on the other side like everyone seems to think it is. I read the book "The Five Love Languages" by Gary Chapman and it helped us out immensely. We've been together for 13 years, but 12 was around the time I started questioning things and asserting that I've had enough of the bad. Things are much better now. He didn't even read the book, I only summarized what the languages are and which ones fit each of us.
I think basically what I got from the post is that you need to quit waivering back and forth b/c that's obviously bothering you. Pick a side and run with it. So do you take the easy way out or do you take the noble way??