Look man, that is a bs excuse! The whole I love you I'm just not in love with you is horse s#%t. If you love someone you have made a decision to do whatever it takes to make it work. Maybe you're not infatuated with her anymore, but so what. You made vows before god for christ sake!! For better or worse...solemnly swear...remember those parts? You, and people like you, are what gives marriages bad names. You better make sure you have done everything possible to make it work before you quit.
I would have a long talk with myself first and ask myself if the source of my discontent was really that I was not in love. I promise you that if you truly love her, that is all we can ask for. Being in love is overrated and that feeling you seek cannot last.
The being "in love with her" are just words that express how you feel in the beginning. You said yourself you love her, so if that's true, your still 'in love'. Its just that, maybe that 12 years, can make you miss that new, charged up, mysterious, head spinning thing that falling in love brings. And you had that, now you have this. if you let this go.....like someone commented - the grass is rarely greener
I'm just going to say this...be very careful what you say on the Internet. My (former) brother in law is a divorce attorney and he'll tell you how easy it is now days to simply print out things said on the Internet and they are 100% admissible. He was representing a wife once and she was claiming her husband has severe anger issues and was fighting for full custody. On the stand the husband was asked if he has anger issues, he of course said no...then a printed copy of his Facebook page was presented, in his bio it said "if you get in my face I'll beat you into submission". Not only did the wife get her divorce and full custody but the husband had to then deal with perjury. Think long and hard as your decision will affect your son also.