It's scary making large sweeping promises, even if they're to yourself. There's something about long time horizons that churn our stomachs a little bit. If I asked you to give up sugar for the rest of your life, could you do it? Just thinking about it might make you uncomfortable. I almost never eat sugar and it makes me uncomfortable. If I ask you not to eat sugar just tomorrow, though, it's easy to agree.
Our brains can conceive of forever and they can conceive of just one day, but intuitively forever doesn't just seem like a collection of single days. It seems like something much bigger.
One of the tricks to sticking to habits is to not think very far ahead. Commit to something for a length of time that makes sense, but then stop thinking about that duration. Just think about today.
I have an ongoing bet with my friend Ben where I can't visit any non work-related site on my computer except during meal time. The duration of the bet is three months, but it could have been any length, really. I know that I can avoid visiting sites today, so that's all I really consider. If a day felt really difficult, I'd just think about each hour. Can I avoid those sites for an hour? Sure, that's easy.