What are
the chances if I say that the relationship is long gone, held
merely by non-existence promise to one another in name of duty to
remain? Duty is inclusive of the lengthy time together as a couple,
which actually amounts to nothing if quality of the relationship is
stale.
Your overseas
trip is not the cause; it is the catalysis.
With a sudden surge of freedom flowing through him, the notion of
cheating without getting caught seemed so ideal. He probably
thought it as companionship, but let me tell you: it is not so
simple. Deep inside him, this relationship is already a lost cause,
for, replacing alternative companionship is but a reason to allow
his conscience to exercise that decision without too much
guilt.
Forgiveness, truthful or even honest confession can take place, but
what is Love left in the relationship?
By now, you probably realize that Love works in a way where trying
is meaningless if you don't know what you are trying for. Tried his
best? Best in what? Tried for what?
Pressured? Need personal space?
Do you see your relationship degenerating?
P.S: Nothing change overnight; just that you didn't know all these
while, changes are constantly taking place. Come clean with one
another in naked emotions - only then you can truely see how your
six years relationship looks like. 
Cheers
