What do I do? Please Help!!!?

I have a friend who is like a sister to me. I had planed to spend the night with her this weekend, but I just found out that I didn't have enough time. We have an Upward Bound trip on Saturday and I will see her anyway. Last time I spend the night with her before a UB trip I felt like she betrayed me because she sat with a guy that she new I liked instead of sitting with me. Any way I made different plans for Friday night when I found out that I was going to be busy on Saturday. The sad thing is that it is with my ex boyfriend, but its not like he will be the only one there. My friend is mad at me because she fells like I am betraying her by making plans with my other friends because I don't have enough time to be with her all day. I spent last Friday night with her, so I don't think it is that big of a deal.PLEASE HELP!!!