Awful situation.
When a person is attacked by someone carrying a knife, it's a common argument that the only reason the knife was being carried was to do serious harm, which most people would agree with.
Her argument in the original trial was quite weak - IIRC it was found that she produced the knife and threatened him with it while they were arguing, after he had assaulted her by grabbing her breasts, then stabbed him in a fit of anger. She said she had forgotten she was holding the knife and had simply meant to punch him (in the chest)...which makes little sense.
Self defense does allow someone to strike first if they feel threatened, however their actions have to be reasonable and proportionate.
Had she punched him, and he subsequently hit his head and died, the outcome may have been different. Had she come across the knife in the footwell of the car while she was being assaulted and lashed out at him with it during the assault because she feared for her life, the outcome may have been different. It may not.
I think - in terms of law - it's probably correct that her defense of 'self defence' wasn't accepted. In terms of her most recent appeal against her sentence, the starting point had already been reduced by eight years in mitigation. A judge in view of all the facts (the key ones being her own admission that her previous rape complaint was not accurate and that the encounter had been consensual) found that he could not mitigate further on the basis of the alleged rape. Again, in law, this seems to be correct.
I doubt she will have grounds to appeal further successfully. This will go down as another terrible conflict of what is morally right, and what is legally permissable.
The only saving grace is that (assuming she keeps her nose clean in prison) she will still be a young woman by the time she is released, and can hopefully go on to lead a meaningful life.