Do your children still live at home or have they moved out?
My view - and this may not be popular, but I'm always being told what a selfish person I am! - is that you can't just leave your home and set up again on your own. Financially you can't afford to, so thats not going to work. I know you say your salary will improve in 18 months or so but you've already waited so long, please dont put it off any longer - you do deserve to be happy you know! And you also deserve not to have to live in one room while the rest of your family is in comfort. Your needs are no less that theirs!
Going back to the house, I can understand you are thinking that it is the 'family' home, you want your children to have somewhere to come at Christmas, birthdays, special occasions etc. Obviously thats what happens now. But if you do move out, do you think that will happen in future? Will you really all still be able to come together under the same roof, or is it not more likely your DH will not want you there if he is? What if he (at some point in the future) meets someone else, and they move in to your old house?
Also what if you get ill/cant work and are unable to pay rent etc on your flat?
In your position I think the only thing you can do is to divorce, and split your assets. Anything else leaves you too financially vulnerable, and on the periphery of family life. If you can sell the family home, presumably there will be enough to buy 2 separate (albeit smaller) properties? you may find it helpful to speak to a solicitor and go through your options - they should be able to provide you with an initial hours advice either free or at a fixed fee.
but please do accept that you deserve to be happy, and that might mean making changes which will affect others. However 20 years is an awfully long time to be in the wrong relationship (I did it for nearly 8 years and 2 years on am still slightly regretful I didnt end it sooner)