Hello motofools & welcome
The thing is, you don't know what happened that night.
She could have been hurt, or ill, or in an accident, or attacked, or with someone else who needed help, or stranded, or anything. Or she may have been cheating on you ~ which seems to be the conclusion that you have drawn.
You say that she was acting suspiciously, and maybe she was or maybe she wasn't, but to end the relationship without knowing what had actually happened may have been unfair on her.
Perhaps you need to find out the truth and not judge until you know it.
Good luck.