Matej is the object, the subject is "you". The verb agrees with the subject, so it can end in -a if "you" is female. Here both nevážil and nevážila are correct.
I am afraid it is not correct. The subject in the Czech sentence is some unknown female who did not respect Matej. I am telling her that she did not respect him and since she is a female, there is an A at the end.
Now, when we translate from English to Czech, we cannot tell who did not respect Matej without context and accept both NEVÁŽIL and NEVÁŽILA
Why is the subject you and not she. I thought for you one would have to use jsi nevazila
The notes say on this topic: “The singular familiar auxilary -s that latches onto host words obligatorily merges with the verb particles, forming the reflexive auxiliaries ses and sis.”
So, first, there is this “familiar” -s instead of “jsi.” And second, with reflexive verbs it merges with the reflexive pronoun “se” or “si.”