Yeah, that happens. Motivation ebbs; expect that to happen. You can get it back though.

Do you like to read? Personally, I find reading helps my motivation more than anything else. Not sure why exactly. Here's a book I like, "Eating Animals by Jonathan Safran Foer https://www.amazon.com/Eating-Animals-Jonathan-Safran-Foer/dp/0316069884

Also a weird pdf of it online too right now: http://cs538523.vk.me/u16932426/docs/a4110b17b1a8/Jonathan_Safran_Foer_-_Eating_Animals.pdf?extra=EPf1ZIWCtZv31RmgkEwacnxt7zeWXvUxJZxz6au_hgsvjWuJFI8amZt5x47Og5pByF7Ga7ttEcw4yX8v4yvD5iGEsZ1wOJ85-f5klA

I also find donating a little money to a good cause serves as a motivation boost -- it makes me feel like I'm part of something bigger: http://www.animalcharityevaluators.org

Also, I see another commenter provided some names of some tasty cheese substitutes. Hopefully that helps.

Most of all, don't beat yourself up. Take some time to appreciate the choices you've made and feel good about yourself, rather than worrying about screwing it up. You're living up to your ideals. That's an awesome thing.