Caught in a love triangle: Boyfriend cheated, promised to change, but still in touch with the other woman

"I have been in a relationship for 8 years, and for 3 years, we have been in a long-distance relationship (LDR). My boyfriend cheated on me with his colleague. However, after some time, he returned to me and tried to convince me to stay. I think he is doing this for his father, who wants him to be with me.

I messaged the girl he cheated with, telling her to stay away from him. She responded, saying she loves him and wants to be with him. She also said that if he's truly mine, he'll come back to me, and if he's truly hers, he'll go back to her.

My boyfriend promised to marry me, but now he's back with me, though not with the same feelings. He promised not to cheat again, but he doesn't treat me well. The other girl claims they still talk because they work together, and he didn't tell me to break up with him.

What should I do now? I don't want to compete with that

girl."