My best friend thinks I'm ignoring her?

I feel terrible. My best friend and I are juniors in high school, and we go to separate schools. So our friendship has been strained and stretched out to just hanging out a few times per year. I am also not a big texter and we have long ago lost stuff to text about. When we do hang out, we are back to how we used to be. We have been best friends since the second grade and totally inseparable through elementary and middle school. I got a job about 10 months ago and it takes up most of my weekends, which we used to hang out freshman year. My phone is broken which she knows. She knows everything but I feel like I am hurting her with having no time to hang out with her very often. She spends most of her time babysitting her 1 year old niece. Today was that baby's first birthday party and I was invited, but my bosses at the last second asked me to work today so I missed the baby's party and I feel terrible for doing that. I absolutely love my best friend. She is like a sister to me and I wouldn't hurt her for anything. We just need to get through the next two years and then we can be done. What can I do to make this up to her? How can I let her know I still care about her and haven't replaced her and am not trying to forget about her?