My first relationship outside of my marriage was a tumultuous one, and my husband used to get angry/upset when I was a mess over it. He felt like he didn't cause the problem, why did he have to pick up the pieces? Nonetheless, of course, he did, and held my while I wept, and was as patient as could be while I healed--but in the aftermath he asked me to try to keep my relationships separate from our marriage. Of course, my moods/emotions are my own and I can't not feel what I'm feeling, but I respect his need for me to be present and focused during our time together. It isn't fair to him if I'm preoccupied or in a funk because of a lover during our "quality time."
I'd say be in the moment with boyfriend as much as you can--and be honest when you can't. The end of a marriage is nothing to sneeze at, it's painful and exhausting, anyone gets that. If you really can't deal with bf right now, assure him you love him, it isn't him it's you, and you will get back into his arms just as soon as you can pull yourself together enough to do so. I'm sure he'll understand.