Choosing our Thoughts VS Being Delusional

I understand we can choose our thoughts. But at what point does it become delusional ? One example I could think of:

A wife thinks her husband is cheating. Her husband talks about a friend that is a girl often. The wife chooses to decide they are JUST friends and continues to be positive and loving toward him. She chooses to trust him.

But what IF he actually was cheating the entire time with this friend of his ? The wife was choosing good thoughts about trusting her husband – so Iā€™m confused by these type of situations.

The thought is not reflecting the reality? Aka the result line ?

C-husband has friendship with a female.
T-they are just friends.
A-remain positive and loving.
R- husband actually IS cheating with friend.

This intentional model does not seem to be serving in this situation.