i know exactly how you feel, i did a 40 day fast last year and lost 29lbs which i kept off for a few weeks. gradually the weight started to creep back on and i'm now 10 lbs heavier than i was before i fasted. i too felt the euphoric feeling one gets from a total cleanse but sadly (due to my own actions), it wasn't to last. i started to eat little bits off "junk" thinking that it wouldn't hurt but before too long, i was back to square one with my totally unhealthy diet.
i have tried countless times to fast and each time i never last more than 2 days, i am slopping around in clothes with elasticated waists and have gone up 2 dress sizes again. i really do feel totally depressed and very irritable but i also know that the only way i'm ever gonna feel better is by cleaning my body out. i'm gonna try and get started tomorrow (1st attempt of the year) and just see how i get on. i know if i can make it through 3 days then i'll have cracked it....it's easier after that. it really is just soooooo difficult sometimes to focus on getting through the first few days. i'm doing juice and so in preperation i have shopped today and filled the fridge full of carrots, cucumber, spinach, apples etc......and also bought litres and litres of water.
anyway the one thing to remember is......however hard it gets, don't ever give up because one of the times you try, well that just might be the time you crack it :)