I agree with previous post, you are not a terrible person. You are not a bad person for wanting to try and find a amicable way to exit a situation that is not what you ever wanted. (except wanting to do the right thing when your first child was conceived).
At the moment though, you are dealing with non related issue's. Try and get them into order. To many issue's which are not related to your marriage can muddle things up and deplete your energy to deal with the marriage issue's. You may not be able to clear the decks of every other issue going on. Who can? But, if possible resolve complication's in your life that are weighing you down.
It has taken you a long time to come to the level of dissatisfaction that you feel and think about your marriage. So, be patient and work on a plan which will cause the least amount of harm for everybody concerned. This might mean talking to a counsellor and bypass all the reason's that you want to exit the marriage and work on how to manage your emotional life and your family as they adjust to the new life you wish to pursue.
It sounds as if you could do a 'trial separation' for instance. At the very least this might give everyone a moment to reflect upon how valued you are as father, husband and provider. It will give you and idea of what 'freedom' might feel like, without tearing into all the realities of leaving 'for good' as a full declaration to all. Avoid re-bound relationships for instance, that would mess everything up and just add another layer of consequence's that you will not eventually be happy with. It takes a lot of strength of character to not fall for that pitfall. But I think you have plenty of strength and character and should be able to see that one coming a mile off.
Be wary of sympathetic female friends who say they knew all along it wasn't going to work out. They are not much help now except to listen to your list of problems and agreeing with you. Sometime's a good friend is someone who challenges your thinking and helps you to clarify exactly what you are feeling and what you are thinking. They are definitely not the same.
You should also possibly consider setting up some counselling for your children if and when you decide to start the 'separation' process.
Maybe your wife will be shocked and angry because if she is in denial about the misery the marriage really is, then she will want to blame. I guess this come's with the process she will have to work through herself. You can try and encourage her to go to counselling with her to see that really, it's got a lot less to do with her and more to do with you. She will feel loss, grief and might become very angry with you. These are all normal emotions so don't be too shocked if she does react badly. Try not to react badly back.
But all of this of course depends on what you decide and how you go about putting in place a plan that consider's everyone's needs and still means you have your freedom and a opportunity to meet your own needs.
It might be a long time before you find the confidence to move into another relationship. I personally would be very wary of looking for that until you have worked out what you really want out of life, aside from continuing to be a fantastic father and a decent man.
It might mean taking up training for new and long wanted vocation. You are probably not as old and worn out as you think. Many people well into their 5th decade change employers, vocation's and take up training. ( I don't know how old you are but with a teenage child you are probably not too old to contemplate anything). It might mean joining clubs and doing the things that you really wanted to do a bit sooner. Obviously, some things might not be possible anymore but you might be surprised by what is.
Are any of these change's able to be made within your marriage? You might be pleasantly surprised by how working on yourself (and being just a little bit selfish) can at least help to resolve non marital issues and who know's improve your relationship with your wife, even if you do end up leaving the marriage.
As for guilt, try not to feel guilty. It rusts out your head and does nothing to help.