Tricky one.
Okay this will be a long reply (sorry)...just don't want you to feel like you're alone XX
My mum told me when I was 30-something that the dad who brought me up wasn't my dad. She didn't tell me in a sensitive way, she just blurted it out randomly one day. Though I was old, own family so didn't really bother me. My uncle is "probably" my biological dad.
She then decided to leave the dad who'd brought me up to be with uncle. Supported her as you get one life, be happy etc.
She then asked dad who brought me up for divorce. Told everyone he'd had affairs (he hadn't, she had). The dad who brought me up was so financially frugal and stable and wealthy. The divorce got so messy. She was violent towards him and took pretty much everything he'd worked for his entire life. Just before he was about to retire. Now he will never be able to retire!
I got dragged into the divorce, from both sides and that's when I started resenting both. My mental health for the first time in my life begun to shatter and it was effecting my own small children and my own relationship with husband. I just wanted to get the hell away. So planned to emigrate to as far away as I could get.
As soon as visa granted to emigrate, mum got diagnosed with terminal cancer. She did want me to still go abroad. So she wasn't selfish. Stayed for her initial chemo and had to handle comments from people in our small town about how I was selfish and neglecting her. I was pretty much rock bottom here. Moved abroad anyway with her blessing. Relationship gradulally improved. She got married to uncle/bio dad just before she got so badly ill and that made her will invalid (unsure if she was aware of this). She then passed away - a few days after I told her I was pregnant :( and 3 days before I was due to arrive back in England to visit her - and everything she wanted to leave me now belongs to uncle-possible dad.
Since she's been gone, I honestly feel every emotion. Anger over will (it's more complicated than I've put on here). Overwhelming loss and love. Confusion. Guilt.
Anyhow, slowly I'm recognising she was just a human. She made mistakes and did what she thought was best. She owed me nothing and I owed her nothing. She had one life and although I don't agree with all her choices, it was her life.
My advice to you is to maybe take a similar view? That your mum owes you nothing. You owe her nothing. Let her live and be happy and you do the same? Just have a lovely and carefree, low expectation relationship with the lady who presumely did what she thought was best for you. Allow her to be selfish. Because right now, my main comfort is knowing my mum lived her life to the full. Money means nothing!!