I'm assuming from your post that you don't have kids? Do you plan on having them later?
Because if you do plan on having kids later, my advice for you is a little different from what I would normally recommend.
Let him go. Period.
Why do I say that? After all, I'm a reconciled DBer, and promarriage, and usually I advocate patience and DBing.
BUT - you're young. It's one thing for YOU to take a risk on your H by taking him back, if it came to that. It's another thing if you plan to have kids. Then, you taking him back means sticking your future children with the risk that one day their dad will walk out on them and break their heart.
Your H has already shown you something very important about his character. When confronted with problems, rather than face them and work them out with you, he chooses to run off to fantasyland instead. Is this the man you would want by your side when a child is diagnosed with leukemia, or when you develop a serious illness, or when you get fired from a job, or any of the myriad crises that can befall people in adult life?
See, I was in your place once. My h was seduced by an ex-girlfriend the night before our wedding (nice, huh?). Six months into our marriage, I find out H has been writing to her and pining away over her. (Five years later I learn that she got pregnant and had an abortion while we were on our honeymoon).
We worked through that over the next year - or so I thought. Really, all that happened was that he started to worry he would lose me, he came back, and I was so glad to have him back I didn't question it.
Fast forward 18 years and 3 kids. H has an affair - because really, he'd never resolved his issues. Truthfully, a large part of our marriage had been contaminated by his "longing", his sense that somehow he could have done better than me. (Totally NOT based in reality, btw, as I can truthfully say I am brilliant, accomplished, loving, and attractive - okay, maybe not modest . But seriously, most of his male friends would love to have had a woman like me).
We've survived his affair. But the pain it caused our children - there are still after-effects. And I realized at that moment, that by burying my head in the sand all those years ago, I had let my children bear the risk that H would repeat.
Do I think your marriage is hopeless? No, but if I were you, knowing what I know now, I wouldn't dream of taking him back unless it was very clear he had gotten help for ALL of his issues. And that would mean at least a year of individual therapy, dating me exclusively during that time, and showing that he has insight into his issues and "gets it".
Would you want to be going through this again in two or three years, with a little baby?
Let him go. Focus on your life. Do all the things you've always wanted to do but were afraid to try. Do all the things you always wanted to do but he wasn't interested in. Find a passion. Travel. Post here and learn about relationship skills.
Your H is far more likely to wake up if you let go and start really living your life. He fars more likely to pursue you if you quit pursuing him. One thing I can guarantee you, pleading and clinging to him will only drive him further away.
So - I climbed Mount Whitney after my h dropped the bomb. What goals do you want to set for yourself?