Can I just point out, *of course being single is easier*. When you're not part of a relationship, you don't have to work on being a part of a relationship. Also, where's your son? He's with his mother. You don't even have to parent. Trust me, there are definitely days where being single is my dream, and my relationship is actually doing wonderfully. Being poly won't alleviate your responsibilities, so I hope that's not why you think it would be better.
Of course, you know you're the only one who can figure out what you want. Do you want the relationship? Then it's your responsibility to communicate with your wife that you want to be poly, whether or not you think she will accept it. You already know what you're going to do if she can't accept it, but if you don't want to walk, then you should at least give her a chance. Make no mistake, introducing poly *will not* fix your marriage, it will put a strain on it. However, I can tell you from experience that in my marriage, introducing poly forced us to deal with issues we hadn't, learn to communicate more effectively, and be honest with ourselves about our feelings whether positive or negative. (myself especially)
Do you want out? Then it's your responsibility to communicate with your wife. I hope that you will be kind, and that you will think long and hard before you walk away. Yes, I think it's better to divorce than stay in an unhappy marriage for the kids, but make no mistake, it does affect your children in a negative way, at least in the short run. There will be acting out, there will be unhappiness, there will be anger and blame. Probably in both the adults and the children. :P
If you want experience, I'm afraid I'm not divorced, but I did marry young. Easy and I were handfast six weeks after we started dating and seven weeks after I turned 18. We were monogamous for 18 years, had two children, and had many issues we didn't deal with, partly because of poor communication and partly because I learned from my mother to suffer in silence. I dread to think what lessons I taught my children during that time. During that time we had poly friends who taught me to hate poly--all I saw was the negative, how hurt people could be by somewhat less than ethical behavior. (Also, one of our poly friends was a zealot, and sought every opportunity to criticize me for "forcing" my husband--who never once came to me and asked to be poly--to be monogamous.) To shorten my story, about a year ago we agreed to be a polyfidelitous quad with dear friends, something I ***never***never***never*** thought I'd agree to. Since then we've had fights, looong nights arguing and crying, but I think overall it's been a positive experience. We're still working on issues, still growing, but I definitely think we're in a stronger place now. Did it come easy? No, absolutely not, it was and still is an enormous amount of work.
Is it worth the work for you? Will doing the work fix the relationship, or do you think that you are too different? Sometimes people's goals are too different to make it work. Can you have something wonderful, or would you two be happier apart in the long run? What would be healthier for your son? For you? Do you love your wife? I think it would definitely be worth discussing why you don't like who you are when you're with her--I would want to know that if I were in that position, but maybe try to phrase it kindly. I hope things work out well for you, good luck.