As an absent mum (legit as it may have been) still leaves us guilty. Telling her you're fine just leaves her thinking that you're a brilliantly resilient kid, not that she wasn't bad.

Looking at this from her side, perhaps being more specific about the reasons behind why you hold the opinion that she's a good mom might help her get a better perspective and be more accepting:

Instead of saying, "but I'm not upset by your absence because you called me," perhaps get into more detail, "You think you weren't ever really there for me, but that time when [insert difficult time/situation] you were the one that talked me through that even though it was over the phone."

You could also tell her that you do not require her to be perfect to love her or to feel loved by her, which is why there is nothing to forgive.