Take is from a guy. Leave him. One thing about broken relationships, is when they happen, it is not an overnight thing. Most likely this has been brewing, and kicked in overdrive when you went on your trip and he had time to figure things out - either on his own, or with his "new love"
Regardless - if he did it once, he'll do it again, as it is much easier the 2nd, 3rd , 4th etc.. time around.
He sounds like a nice guy, however from a guys point of view, he probably has a change of heart - and doesn't know how to break the news to you - some of us guys are like gorillas and are just not that versed in expressing these kinds of things. A few grunts and we think we got it covered.
Love should be natural - much like that of your children. YOu have to ask yourself, does your children do anything that requires you to "work on your love for them?" of course not.
If you have to "work on it" with him (and he you) you are not with the true love - only a filler. Sometimes they are long, other times they are quick and painless. but regardless - love is not something you work on. That's just my opinion.
It is what you feel in each other. Its is just sad in life that we meet the wrong people all the time, and the sparks fly, but in the end, one person fizzles out, and doesn't have any fuel that the other can light them with.
Even if you work it out, you will never trust him again. You will always be wondering. (i know this from personal experience) - There will always be a hole there.
but guys are a strange breed - sometimes we think we want someone in our lives, and after its there - we're not so sure anymore. some guys like the chase - sometimes they realize it AFTER they got you, some are not so sure and hang around - Yes it sucks - but it life. Look back on your relationship, how many times does he seem content with you gone on even the SMALLEST of trips (church? shopping? etc..) and does he stay at work longer sometimes? Those might have been hints.
I've been on both sides of the fence, I can give you a good guys point of view. I'd like to hear how this pans out. But trust me, if you feel like you got hit by a truck - he was not being the least bit open with you, and that is not fair.