When deciding whether a charge should be 'ABH' or 'GBH', Crown Prosecution Service staff are obliged to refer to the guidelines in this document:
https://www.cps.gov.uk/legal-guidance/offences-against-person-incorporating-charging-standard
As I read it, there can be no doubt that the appropriate charge should be one of 'GBH'.
When seeking to defend yourself from an attack the law permits you to use 'reasonable force' (when you can't simply avoid the attack through running away). There is also case law which is based upon the words of Lord Morris in a 1971 court judgment:
"If there has been an attack so that self defence is reasonably necessary, it will be recognised that a person defending himself cannot weigh to a nicety the exact measure of his defensive action. If the jury thought that that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought necessary, that would be the most potent evidence that only reasonable defensive action had been taken ..."
So you've done absolutely nothing wrong.
There's plenty of psychological research which shows it's not at all uncommon for victims to (wrongly) blame themselves, in part at least, for offences committed against them. So the thoughts and feelings you've been experiencing aren't really unusual. (Many people have them). However you need to keep telling yourself that you were WHOLLY in the right and your attacker was WHOLLY in the wrong (with no 'grey areas' at all).