PLEASE HELP!!!!WHAT SHOULD I DO? IS THIS GOOD OR BAD?

Ok so i have been full fleged dieting since june and lost a a total of 125lbs so far.I am 15 and have a severe calorie restriction problem.So this weekend is my sisters 21st b day and i already knew therer would be alcoholic beverages and yes i know you say your only 15 but my mom lets me do it and i am very smart with how much i drink and how i act. so i am completely worried about it all week because im worride about not exercising and gaining weight from that plus the alcohol i planned on drinking.But i didnt expect the food to come.I had planned on just eating my normal healthy foods.But having a drink after a whole day of not eating turned into me completely starving and the first words out of my mouth was TACO BELL so i had a nachos supreme and a chalupa supreme followed by a large Dairy queen reindeer bites blizzard and i ate it ALL.I feel like im such a fat and i feel horrible.Only worse is tomorrow is when i was planning on eating not the best because thats when we were going out to dinner for her b day and my sister had already agreed on applebees so that i could have something of the weight watchers menu and i was fine with not eating all day so i could enjoy this meal even though it was a good choice.So my sister really had been wanting to go to this mexican place for her b day but being the good sister she was she wanted me to be able to eat to but guess what else happens in this night....I go why dont we just go out to mexican i dont really care if i gain a few pounds this weekend after a 3lbs loss this week.So know after a little while sleeping the liquor off and waking up feeling sick from the food and completely guilty i ask myself what should i do about tomorrow....not eat at all or eat and feel more guilty.My mom kept telling me all evening that i deserve a weekend of rest from exercise and certainly deserve of weekend of splurging and may i tell you i didnt even eat on thanksgiving at all so she keeps saying youll be fine you dont eat enough and exercise to much.But i feel so afraid of gaining weight. I weight 140lbs and i eat a total of maybe 500 calories a day and walk 5 miles a day through a workout vid and walk 7 blocks a day and not to mention i do a exercise bike 40 min a day.What are your opinoins on this!!!Please help!!!I just need somebody to talk to.Will this ruin my progress???Should i feel guilty after dieting so restrictivly for so long and exercising everyday and i havent gained any weight at all since ive started but constantly lose every week.Please just help!!!Kindness is needed!!!