I slept with my best friend's mother before she passed away and I feel hertbroken about it
That's a lot to take in. Which part do you feel bad about?
I feel bad about having had an intimate relationship with her.
It's good to feel remorse, but you should never sleep with a friend's parent. You should form relationships with people closer to your age.
Blame me. All I have are regrets.
As long as you are aware of what you have done by sleeping with them, I believe you can properly move on. This mistake is a part of you now, so I believe it's better to not keep clinging on to the past. How about unloading it with a therapist?