I agree, keep trying. It may take her a while to adapt her sucking pattern to breastfeeding when she is used to the bottle. It is wonderful that she was even willing to try latching on the first time you tried after a couple of weeks. I think that is very encouraging.
Do you feel your milk let down? If so, this will help you know when she is really getting milk, in addition to listening for her suck-swallow pattern. But not everyone feels let down (I am one who doesn't), so don't worry if you don't feel it.
The fact that you could still pump 80 mls doesn't necessarily mean she didn't get milk. It's not like you completely empty them out and then have to wait for them to refill--you also keep producing milk as you go. Can you tell by the feeling whether you are still full? I would think you would still feel full or engorged if she didn't take much. I don't have any personal experience with mastitis, although I have had plugged ducts.
It is perfectly fine to keep expressing, and every drop of breastmilk you give her is wonderful for her, no matter how it is fed to her. I just think it would be wonderful for YOU if you could get her nursing and not have the hassle of pumping. But do what works for you. Whatever you decide, I think it is awesome that you have done as much as you already have.