There is a very old fantastic song called "You have lost that loving feeling" by Righteous Brothers.
The lyrics clearly state the situation and requests the girl to love him back again.
A heartbreak of a situation, more of a slow death actually.
What if we lose that loving feeling of something or someone that interested us much but lately, you have started losing that loving feeling? Not because you found a better love, but because the other thing/person disrespected you again and again despite you putting all the efforts and the love for it.
The feelings are depressing and sad with a pinch of anger (not sure about the other or towards self for loving and doing so much so far along).
The thoughts slow down, the body feels weak with the weight of the negative emotions including missing a few good things and simultaneously lightness of the burden of responsibility which is going to come down.