I agree with what somebody said earlier about taking some time off. If you usually take Saturday and Sunday off, throw in Monday and Tuesday one week. Go visit Universal Studios in Orlando. As an airline pilot, I started feeling the same way not too long ago. I no longer cared to go to work any more.
I took a short vacation and realized the importance of having some "me" time. It did wonders, and I realized I was simply flying too much. Now I try to have a schedule where I get more free time. I take a slight pay hit, but I'm much happier when I go to work now.
I realize that this may be a bit harder as a CFI. Not sure what your setup is, but as an instructor, if I wasn't teaching, I wasn't getting paid. No such thing as monthly guarantee. In that respect, it may be harder for you to financially justify taking a break, but an extra day or two off every month can really make a difference in your level of enthusiasm.