I would not be able to forgive a partner and would have to break up with him/her if...
Я никогда не смогу простить любимого человека и расстанусь с ним / с ней, если он(а)...
cheated on me
he lied to me about something very important
he was in love with someone else
he was unfaithful, started loving me less, put his own interests constantly above my own feelings, or stopped valuing me.
I never give a second shot to anything. If circumstances allow it, I usually remain in good terms with most of my ex partners, but I never enter a romantic relation with them in the future.
she broke my trust
she lied to me about something that's very important to me, or if she turned out to be too self-centered.
she was unfaithful to me.
they did not respect me
they hit me
they hurt one of my friends or family members.
would cheat on me with someone else, although if it was just one time, I might be able to forgive him.
будет сильно меня ревновать или не доверять мне.