On the vegan part; you aren't less of a vegan if you accidentally eat meat. The identity is your intention, not your perfection.

I find that not doing something is relatively easy (like don't eat meat or stop alcohol, both of which I succeeded in), but starting something or gaining a new "habit" is damn near impossible. Basically it's the OP's scenario. Get super excited and way into whatever thing (go to the gym, study, get up early) then 2 weeks later it falls apart.