How to Forgive Myself?

So my mother passed away a little over a month ago, and we had had an extremely close relationship. One of my biggest regrets in life though is that I once made her cry. I didn't mean to. I kept apologizing and I think she forgave me (I just don't have a good memory. I'm sure she did?) But I still think about that time constantly.
There were only like 2 isolated incidents where I made her really sad from something I said that I shouldn't have said. It wasn't like, "You're the worst mom ever!" or anything.. But I'm also wondering if part of her crying was just that she was emotional that day? She had cancer and fibromyalgia for years and suffered physically.
How can I learn to forgive myself? I feel awful. She was the world to me.