I don’t understand because wasn’t Olaf’s actions of lighting the fire, almost melting and then trying to save her and act of true love and shouldnt that have unfrozen her heart?
Yes, but didn’t it have to be a kiss that broke the spell?
No because the kiss would never have worked. Olaf is performing an act of true love, but having an act of true love performed wasn’t what would thaw a frozen heart, you have to perform one yourself — that’s what this scene is proving. The kiss was a red herring, the act of true love that saves Anna is Anna deciding to save her sister rather than herself. At that point Elsa could have screamed “YOLO!” and run off into the sunset and it wouldn’t have made any difference, because Anna has true love in her heart now and that’s what ultimately thaws her — she loves Elsa completely unselfishly, whereas most of their previous interactions have been all about Anna wanting something from Elsa.