How should a guy act in a long-term relationship that he is not sure of anymore?

The question above has nothing to do with love. Love makes what 30% of a relationship and there are many other things to consider. You can love someone with all your heart but if they keep hurting you, that does not mean you should remain in the relationship because you love them. Or if there are trust issues you can't get over , then there is no use being in the relationship if you've tried to overcome them in vain.

All I'm saying is look deep into your heart to see if what you thought was love still exists and then use your head, to figure out if there any obstacles to the relationship u cannot handle anymore or that can be worked out. Then make a decision, the sooner the better, if it is worth it to stay and make it work or to leave for the better of you too. Communicate these issues too, just don't start acting weird. Yep, making decisions are not easy!! especially with matters of the heart, but they have to be done!