We did have to work at it for a while. There were things that really pissed me off, not least after my DS was born (and the run up to it). It was born from excitement mostly, and not understanding the boundaries. She had never had to share anything, DH's dad being out of the picture since before he was born, so she called every, single shot. It came across that she was baffled that she was being forced to give up some power now.
What's helped has mostly been time. She adores the children, and has also has come to terms with the fact that they're my kids, they're my (and DH's) rules. She thinks they've turned out well, so she's sort of accepted that I'm doing OK.
She's as tactless as they come. She also will react instantly to a situation before going away, talking or thinking it out and deciding what she actually thought. Point in case, my being diagnosed with bipolar disorder. Instant reaction; well you can't know how other people feel, so how can you possibly know that you're different? You can't compare yourself to being in a wind-tunnel because you've never been in one! (Ditto depression or any other mental illness). She said she thought I took far too much medication.
But...
Within a few weeks, she'd read up, started asking me questions, asking what help is needed with the kids etc., and even has recently said that she's impressed at how well I manage my medication and the situation as a whole, and the way in which I discuss it with the children.
So it's not so much that I have to forgive her. It's more that I have to wait until she gets her thoughts in order, but yes, sometimes those things can sting.
NB: I'm no walk in the park either and can really dig my heels in and return barbs that come at me.