You shouldn't stay in a relationship with anyone you don't trust.
Change in behavior is usually the biggest cause of suspicion. Generally speaking it's behavior that one dislikes. Nevertheless if you found out your mate was NOT cheating and the behavior remained it's unlikely you'd jump and down with joy.
Cheating would have simply given you an "explanation" as to why their behavior changed.
In some instances people (need) their mate to cheat or be abusive in some way in order to give themselves "permission" to end a toxic relationship or one that they've been unhappy in. Simply being "unhappy" is not enough of a reason for them to walk away.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde