I've screwed up with my bestfriend! please help me! she's gonna leave for college :(?

ok my bestfriend and i have had our fair share of fights and arguments. i have two bestfriends crash and kay kay pilot we're 17 and 18 years old i'm the youngest and kay kay is the oldest. ok so this is the deal. we're not spending enough time with eachother. she says that everytime she asks us to do something that we end up not doing it. and it is partly our faults. we should always do stuff together. things always seem to come up when we're trying to do something together and that pisses her (kay kay pilot) off and now we're definetely not ok. i guess i'm beeting around the bush here but idk she's mad because she's going off to college this year and she says that if we can't do anything now that she's still here that when she's gone we'll never get to do anything. she's my bestest friend and god i love her. i hate it when she's mad at me more then when crash is mad at me. she's different then anybody else. i look up to her. and lately i've screwed up with her by ignoring her which was only one time and then another i never saw her but she thinks that i did and that i just ignored her. but anyways i'm gonna apologize to her and i just don't know what else to say to her. please help me. i've hurt her by not being with her when i said i would i didn't meant to blow her off things just come up. so please what should i tell her. what have you and your friends been through? and said? i don't want her to leave mad at me. P.s. what are some ideas on what i should do for her to forgive me? i was thinking about writing her a nice letter and putting it in her mailbox...any ideas?