Does your boyfriend or girlfriend have a wandering heart even though he's/she's with you? How do you deal with it?

Being with a guy/girl that is supposed be your boyfriend/girlfriend and a couple with you should make you feel secure. But when you are with a guy/girl that has a wandering heart, you really can't relax because you can't trust them - right? What to do?
Does your boyfriend or girlfriend have a wandering heart even though he's/she's with you? How do you deal with it?
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