Rhetorical question I suppose, but also not. DH and I have been 'separated' since September although have been living together again since 1st November (separate rooms). We have been v clear we are on a break, ie not together, not socialising together, not really spending time together etc. Have missed him but wanted to give it a proper break to decide what I/we want for the future.
Since separating I have had a fling. It has been purely sexual and made me feel myself in a way I haven't fot a while, recapturing my youth or some shit. (I'm aware that none of this is very mature or helpful by the way, which is why I haven't posted on here for advice because your advice would probably be on the lines of 'you are a pair of immature twats' but hey...)
I believed that DH was also shagging someone (wouldn't surprise me). I have snooped in his phone (I know, I know) and seen texts from/to a girl, nothing overt, but clearly more than friends. He usually deletes them all as well (clue...)
I was hypocritically upset about it but kept it to myself, he has the right. Then my best mate put the wind up me by challenging me over things and pointing out that DH might actually fall for her and shook my complacency that we were just getting something out of our systems and would come back together in the future.
We had a conversation in which DH hinted he had also snooped in my phone and was upset that I had had something physical. I pointed out that so had he, and he denied that he had so much as kissed her. Half of me doesn't believe him, but the other half thinks 'so what'? Having looked at his phone this morning after he fell asleep early last night and didn't delete some texts it is clear that this girl likes him and I think he might like her too. This upsets me far more than the idea of him having random sex. He has spoken to her about the possibility of him moving out of our house etc.
Fuck this is an essay, and I only wanted to ponder which was potentially more upsetting, what I did or what he did... I have namechanged because I don't want people to remember this and so if you flame me I can pretend it happened to someone else 
So please do answer the original qu, or call me a silly immature bitch if you like, and pronounce doom on my marriage....
FWIW I do want it to work, but DH was completely unappreciating me and taking me for granted. He also had some very entrenched behaviours that I will no longer live with. Not saying I'm an angel but I was pushed to limit. He is making an effort on his own account to deal with behaviours and I'm (I was) somewhat positive that we could get through this. Not sure now....