OP, I haven't read all four pages of your replies, but have read a few and lots of people who are heartily agreeing with you seem to have had much less stable marriages than the one you describe.
I think it's normal in a long marriage to have periods of falling out of love. But you can also have periods of falling back in love, unexpectedly. So it's not always wise to ditch a marriage when you first notice the love is gone, especially if that feeling is one-sided.
I wouldn't split up a family when DC are in early teens, for something as trivial (imo - I know loads of people will disagree) as being 'out of love' with a calm, stable, trustworthy long-term partner who is a good dad. Sod that. I'd work really hard on putting some passion back into my own life - not sexual passion, but passion for life itself. And give your DH a kick up the backside to do the same himself.
Get DC to draw up a list of really exciting things they want to do as a family - places to go or experiences to have. Talk to DH about what skills you want your DC to have by the time they leave school at 18 and set about helping them to learn them. Making yourself take an active, renewed interest in family life and in your own life as an individual can get you to appreciate each other again.
I'm not suggesting people should always stay together for the sake of the DC, but if your partner is a good man and a good father, it's a selfish thing to do while DC are just entering the rocky time of teens with exams and hormones looming. I think it's a bit cruel, really, to split up their family home and stability just because one parent is looking for a bit more oomf in life and thinks they have to destroy family life to get it.
So, I'd advise you to get really focused on a handful of things you long to do, and do them. Then, if, when DC are grown, you still want to leave, you'll be a fitter, more successful, more dynamic, confident and attractive person for having achieved all your goals while raising them.