For quite some time I have struggled to forgive certain people in my life for ways they have hurt me in the past. There have been times when I do forgive them, and even feel great compassion and empathy for them. Yet the feelings of anger and resentment return. These feelings have gotten to the point that they really are harming me, causing me to lose sleep and so on.
Does anyone have advice on how to forgive and resolve these feelings so I can move on? I'm not a theist so God-based replies won't be particularly useful for me.
Forgiving where it isn't easy, the harder case.
While like you I had the basic idea (that's been around in psychology articles for many decades) that forgiving is a very beneficial thing for oneself -- you escape the weight of what they did to you -- I went further after I was intrigued to read the message from a far older source. (I was systematically learning and testing things from around the world, and across many ways to live life better (to find the best ways so I'd have the best, and get an even more rewarding life), including psychology, philosophy, religions, techniques, and so on.)
And so when I read what the teacher Jesus said to "forgive your brother or sister from your heart", that was the bigger impetus to try it out in that more difficult case, with the less forgivable offense.
Here's what I did.
What I did was try to think of that person that had done wrong towards me as themselves like
only a wounded child (in an adult's body) who was only acting out of their own pain/confusion/lostness.
But they were really just like a child inside.
So, by realizing they had done wrongs even in some parallel somewhat like (even if lesser) wrongs
I had done in life ...
In other words, they were like me, in that very rough parallel (different, but in a way similar, or what I might have been on another path).
Then, thinking of that, I was from that thought then able to forgive that harder case, and just accept them as a victim, and even feel some sympathy for them.