Need serious advice for an apology

So what happened few months ago is that I caught feelings for my bestf and obvio she realised it from my change in behaviour. She was against it so eventually we ended up not talking to eachother without discussing it as the reason was clearly visible. We were too good friends and she had helped me a lot whenever I shared my personal problems to her. She's literally the purest lady, I respect her alot but don't know how and why I ruined our bond. I should have tried to save it but no I am a selfish rascal.

But as the year is ending I want to apologise and say thanks to her once. Please tell me what to do...