How can I open up to my mum?

First of all, It is great that you want to open up. I feel that by simply telling your mom that you want to talk with her, is a great way to start a conversation. You can also respectfully state your expectations from her. For example, you can say, "Mom, I want to be more open with you from now on, because I want to improve our relationship, but I also want to know that I can trust you to be understanding." You can also discuss together the possibility of creating a "safe space" time in which you agree to openly discuss any pressing issues responsibly and without judgment. Hope this helps!