Need advice please :[?

so recently ive been feeling like ive been going under depression. this weekend i was supposed to finally go see my sister in another state and have some freedom for a couple of days. it now turns out that my dad doesnt want to take me anymore. so i had another opportunity to get out of the house because my older sister was going in a couple of days. but when i asked her she told me no because there were going to be to much people in the car. and i know shes lying because theres only going to be 2 other ppl with her. ive been looking forward into finally going some where and being free instead being stuck in my house because of my very strict parents. ive tried asking her again and she still wont take me nor will my parents. i have been crying this whole week because of this huge disappointment. i was wondering if anyone knew why this disappointment is crashing on me so badly? why cant i seem to get over it and stop crying all the time? i still have a whole month until school starts and i have no clue what im going to do with myself. im just so sad and depressed all the time.... any advice?