hmm, I think after all those years with Thien, she got the message that Thien would never love her, so she wants to have some good memories before she leaves for studying aboard, which is also something she wishes to do. She was ready to forget him but in Myanmar, Thien fell in love without noticing and began to treat her differently. There's probably no exact reason why he loves Muey, because sometimes love just can't be explained. Thien probably has feelings for his junior but did not know that the junior was Muey, and with all these years living with Muey, Muey is already part of his life. Then he found out the truth about Muey willing to sacrifice herself to save him, which made him very guilty and shame for not noticing her love and cherish it. So he left to give her freedom, let her go to study aboard, and sign the divorce paper which is what Muey was asking for.
On the other side, Muey already misunderstands that Thien not divorcing because he doesn't have money, and then he just left all of a sudden, with just a letter. Then she found out she got pregnant, which stopped her from going to study aboard. He then showed up all of a sudden again and not giving a persuasive explanation of why he can't leave her. Now she will think that he came because of the baby and misunderstand that he wants to be with her because she saved his life. She knows that she loves Thien very deep, and she's trying very hard to forget him by moving to a very far place and block him and his friends from all social platforms, but Thien shows up which makes it difficult again. With all these reasons, she was very mad when she saw him. Like she said, she wants like a 2 years gap of not contacting each other to see if they actually love each that much, she really wants to know if they both actually can see their heart clearly.
I think the p'ann and p'aew did well on changing this plot, and I actually like this change because it does make sense to me, there is a reason behind every action the character made.