God, here we go:
I tend to get hit with apathy at least once a month, and I get hit hard, meaning it can be difficult to do or find enjoyment in anything.
I find that usually the best thing to do is slow down, take a step back, and reorganize your priorities:
"Yeah, I've got this big project, but it's dragging me down, and I can get this done quicker."
In order to combat that unmotivated feeling, it can help to find a few, smaller projects you can work on and quickly knock out, to get yourself up and running again.
If that doesn't work, then the best thing to do is re-energize yourself, eat something you enjoy, take a nap, enjoy a nice hot shower, etc. Self care should always be at the top of your priorities, and sometimes just doing some stuff for yourself is the best way to get rid of that feeling of a weight in your stomach.
I know this isn't the best advice, and I don't pretend to be a professional, but I want to help other people struggling with apathy like me.