The best thing to do when you feel that way is to take some time off, and mix it up a bit, go for some dirt rides on your mountain bike or cross rides or whatever. Don't check speeds or heart rates, just spin around. And if you don't want to go, don't go.
I sort of cooked myself yesterday, so I may take today off. It's supposed to be 90 degrees (that's 32 C) almost 100 with the humidex. It's either a sunset spin or nothing at all.
Anyways, taking a few days off or doing different rides with no pressure, no goals, should get you back to being motivated again. I'm not feeling the blahs yet because we had a really horrible, rainy spring and then I missed abuot 10 days with an injury, so I'm still really motivated. However, on brutally hot days like this, the hell with it!
You can catch more flies with honey than with vinegar. That's great...if you want to attract vermin.