Why does happiness often feel temporary, and how can finding purpose change that?

I (f) almost kissed my best friend (f), what do I do? I like her but I'm not ready for a relationship, and I need to take things really slow, but she's the kind of person who wants more now.
I promised my girlfriend I would marry her, but I broke up with her after 2 years of cheating and red flags. Am I in the wrong for breaking my promise?
How do people successfully adapt and grow together when their individual desires or life circumstances change significantly during a long-term relationship?