TRUST

You need that for a relationship to work. Being able to not be there but not be worried that the other person would do anything wrong. 
To trust their every word and to know that, even if they're keeping something from you, although you really want to know it, they're doing it for your benefit.
Give them the benefit of doubt. You might have questions but you trust them enough to know that they won't cheat on you, won't lie unnecessarily to you, won't betray you. 

To be suspicious. To feel agitated and to try and figure out what they're doing, that's not the way to go. 

if you can't do these things. if you think it's too hard for you, maybe you shouldn't continue with that relationship. maybe you just don't deserve them.