it was good until the ending chapters where her heart would start racing for other people

Honestly, I think that's normal. Most relationships go through "cooling" periods. Those who stay together choose and work through it <3
A handsome or beautiful person will get anyone's heart racing, depending on your sexuality. I find it more admirable when couples are able to be honest about stuff like that without being insecure or obsessive. Shows they have full trust in one another :>