Thank you all for your help and responses. The one I hurt was not a human, but a canine. i guess I should have said that.
Now that she is better, it doesn't seem like such a BIG deal, but I am still feeling guilty. I gave her a bath and then put on flea medication. I guess I must have got the flea meds in her eye, because a few hours later, one of her eyes was swelled shut. I freaked out! i thought I had maybe blinded her.

I had some saline in the house and washed her eye out for a good while. She seems better now. Her eye is open again and I know she can see out of it, because I covered her unaffected eye and she followed my finger with her other eye.

She is going to be OK. I just hate it that she suffered because of my mistake. I was trying to help her, but instead I hurt her. I HATE that she felt pain because of me. I HATE that she can't understand that I did not mean to hurt her. All she knows is that the one she trusts more than anyone did hurt her. How confusing that must be for her.

I have done my best to communicate to her that I love her and I am sorry. I have no idea if she understands that or not.

But the important thing is that her eye is better and she is healing.

I hope that if she felt betrayed or abused, that she will heal from that too. She is such a sweet little girl who has comforted me many times.

I am going to have trouble forgiving myself for this, but maybe, with your suggestions, it won't be one of those things I hold over my head for the rest of my life.