Mum almost 50, I'm 26. I can't work out if this is her being malicious or she just has a really bad memory.
She will often offer me a nice favour, or say she will help me out with something, say childcare or money. We can have multiple detailed conversations about it previously (I'm always very thankful and never expect anything of her) but further down the lien when it comes to doing the favour she will deny the conversations ever happened, saying she wouldn't offer such a thing, etc. I didn't realise how demoralising it was when I was younger, but the longer it goes on it just feels like I'm being gaslit all the time. Her memory isn't bad when it comes to other things.