Mum passed 5 days ago

My mum passed from lung cancer 5 days ago. I’m 20. I am really struggling to cope and can’t help but think that she must’ve been so lonely in the months leading up to her death because I left her for university. At the time I was unaware it was this severe, as she kept it hidden from family until she was admitted to the hospital. I wasn’t there for her last birthday or her last mother’s day. The day before she passed she couldn’t even talk, but she grabbed my hand as I was leaving and I pulled away. I miss her so much and feel like I let her down. I don’t know what to do.