You do seem too sensitive about it and maybe pushed yourself into going near her that one time on her bday when it wasnt really comfortable for you. If it were me, if i felt i wasnt going to be able to speak up to someone cause we didn't click well I wouldnt bother or care that they went their way and I went mine. She started 'avoiding' you as you call it because you guys werent on the same page so why bother keeping up some awkward thing. You said you eventually just kept quite around her.. so its a lot more of a matter that she just moved on to have conversation with others rather than was purposely 'avoiding' you. Did you really want her to sit and stay by your side when you just didnt talk to her anyway? That's not fair to her or you. She was better off going her seperate way. and its not bad that you didnt care for her. You will meet people you like who feel the same back.