I hate disappointing friends

Recently I have been so bogged down with class, work, and having enough money to do gifts for everyone for Christmas that everything else just gets put aside. I try to be the best friend that I can be, and I am always here to listen, but I do not have a lot of free time for my friends and they sometimes feel that. I have one friend who is trying to do a book study over a women's christian book, and her first one was supposed to be this Thursday. I think I was supposed to be there, and she was expecting me as her support, but I failed to ask off of work to make sure I made it to her book study. I had to ask of the days before though so I could have time off work to study for my finals, otherwise I would try and get my schedule switched, not to mention I need to work to have enough money for Christmas. I feel really awful about disappointing her, and she does an extra good job making me feel worse about it, but I try to stay positive and not get mad at her for being a little over dramatic about it. And that is horrible I say that, but you all know the time when you know people are upset with you and text "Alright. It's fine." I do not do well with people being disappointed in me, and it really eats away at me. I try not to think about it all the time, because she gets upset with me quite a bit, which I do not like at all. But I mean you can only apologize so much and feel bad so much. And that never solves the problem, so I need not to worry about it so much. I hope she will forgive me soon, and know I did not intentionally forget to ask off. :/ I hope this blows over soon.