That night, Finn said he wanted to start over with her.
She rejected him at that time.
It was true. Since she had decided not to be with Finn, she did not want to be entangled with him anymore. In fact, she had her own bottom line to keep.
When she rejected him, she knew how much her heart ached and how hard it was for her.
Hence, that night, she really cried buckets and went to work the next day with her swollen eyes.
However, Finn showed no emotion.
She admitted it. If Finn could be more persistent, or if he said it a few more times, perhaps she would compromise.
Nevertheless, Finn would not. If she was rejected, there was no turning back.
In that case, she did not want to stay any longer. She would never be able to get Finn to give too much to her anyway.
From then on, Monica really accepted Michael, from just saying it to accepting it.