It depends on the circumstances of course.
If your relationship before he left was good,
but he instantly became infatuated with someone and decided to pursue it,
in spite of your response,
then you have no choice but to let him go.
after the honeymoon phase has worn off,
especially if he rushed into it.
If you want him back,
focus on living the best life you can,
building up your own friendships and interest,
dealing with any issues of your own that might be holding you back,
and keep in friendly occasional touch with him.
Don`t show jealousy,
He wanted some freedom,
not from you,
but to find himself in mid-life,
and you have a choice;
give it to him and forget about him;
force him to stay (not a good idea unless there are children);
give it to him and live your live but keep the door open.
Flaws are bound to develop in the other relationship,
and the other person may well not be so infatuated,
and may fall of that pedestal.
You have got to be courageous,
be open to the possibility that he may not come back,
or if he does it is for friendship alone,
but on the other hand,
if the relationship was good before,
the fact that you have both had time to deal with some things,
may well improve the situation if and when you find yourself meeting him again.
Then you need to go slowly,
and listen carefully,
because he`ll have changed.
Maybe he`s steadied himself;
maybe he needs more time;
maybe you`ll realise it`s never going to work.
But if you decide to give this a go,
you have to have the right attitude or you don`t have any chance.
Attitude counts for such a lot..