怪我心小只能爱你一个
[guài wŏ xīn xiăo zhĭ néng ài nĭ yī gè]
A direct translation of '怨我心小，只能爱一个人' would be ‘Blame my limited heart, capable of loving merely one person’. In short, it means someone can't love two people at the same time, and this bothers them.