Well, how long?... how long indeed....
You need to make a solid commitment to yourself, to your marriage to try and make it work. My initial gut feel is give it to the end of the year.
It's true a good companion all by itself does not make a good marriage. I think you and DH need to honestly sit down and address the issues. Tell him how good a dad he is, tell him how you do enjoy his company. Tell him all the good points about him (otherwise he could do that blokey thing of just giving up and withdrawing altogether, defeating everything) If you still ideally would like a good shag with him but circumstances have put you off, TELL HIM! It's important for him to know you still want to have sex with him.
I wouldn't worry about OM, really I wouldn't. If that is meant to be, it will be. believe me.
I truly think this guy is a catalyst for what you are missing. i think, honestly, that if you were to ditch DH today, and run off to the sunset with the OM that it's highly likely that the new relationship wouldn't work.... There is a lot of work you need to do on your marriage, and then on yourself before that will just slot into place. You need to end one story before you start another.
It might be that you can't make this work. It might be OK and you do get back on track.
How OLD are you? have you recently celebrated a milestone birthday, or are you approaching one? This could be significant too.
I think you need to sit DH down, be honest with him, you don't have to tell him about the OM, just tell him that you found yourself wondering about how it would be like in other scenarios etc and you are taking this as a warning, a wake up call - which it is.
What you are missing is not necessarily the OM, it's what you had with DH, what you are missing is what IS missing in your life now. OM is only a person giving you attention, or being there for you, when your own DH isn't, for whatever reason.
Cut all ties with the OM, you need to, for your own sanity, and so you can commit yourself 100% to saving your marriage. Both of you have to be committed to saving it, otherwise it just won't work.
I know you say about when do I get to put my needs above DH/DC, well in a way you ARE putting your needs first right now, by trying to save what you have, by trying to keep it all together. If you don't try, honestly and earnestly, you will never know. If you try and it doesn't work, you won't be the one kicking yourself. That, believe me, is important.
Try relate, don't worry about the sex for now, they can help you with that too.
Now, best foot forward young lady and into battle, give it your all, give it what you got and don't be distracted by anything. Then evaluate at the end of the year.
If it doesn't work, then you have to plan what to do next.
What comes next is some YOU time, time ALONE, to re-find you, to regroup, grieve and heal. None of that essential work can be done with anyone else in the mix.
We are with you every step of the way.