Rekindling an old friendship

Two years ago, I had a really bad fall out with my best friend at the time. She was really struggling with depression and I was doing my best to support her, but I started going through my own struggles (my grandmother got sick and passed away, my mum was diagnosed with a chronic disability, and I was suffering several chronic health problems myself). I was in regular contact with Childline to get advice on our friendship, because I loved her very much but the responsibility of being her only person she could confide in and rely on felt like too much on top of all the issues I had, and I felt really guilty because I felt like I was being selfish. Anyway, she went off to college and I stayed at my school's sixth form, and we had a really bad argument about the fact that I made some new friends in sixth form that she didn't approve of. We sort of lost contact a little because I kept cancelling plans with her, mainly because my mental health wasn't in a good place because of my family issues and I couldn't deal with her problems too (it sounds awful, I know). She thought I kept cancelling on her because I'd ditched her now we didn't go to the same school, and it turned into a massive argument. We both left it a little too long to get back in contact with the other person, to the point where we went past 'cooling off' and when we eventually did get back in contact, we were both angry that it had taken each other so long. We ended up just cutting ties and I haven't spoken to her since. We've finished school and college now I assume, and I've been wanting to reach out to her for two years, but I never knew how to do it. I was always worried that getting back in contact with me knowing all the problems we had would be detrimental to her mental health- she had a new group of college friends and seemed happy, and I didn't want to disturb that. She has new best friends now, and it seems as though she's more confident on social media now than she was before. But I still feel our friendship deserved a better ending, and I want to get back in contact with her, even if it is just to end things on a better note. I recently added her on Snapchat but I have no idea what to say to her that will send the right message. Please help x