How do you let go of hard feelings?

I just found out that my "uncle" died and I'm feeling some resentment towards my mum. He was very old and ill so it's not unexpected but my mum was the only point of contact I had with him and I never saw him as often as I wanted.

His wife was my grandmother's friend and used to be like an older sister to my mum. When I was a young child him and his wife both moved to the area for retirement and had some hope they'd see my mum a bit more but she had 3 young kids at the time and honestly told them it wasn't likely she'd see them much as she barely saw her own parents. My mum thinks it changed things that she said that because after that it was always difficult to see them as they made it clear she couldn't just turn up on the doorstep and had to arrange beforehand and she got angry with my mum a couple times over things that really weren't big deals (like dropping by with flowers and planning on leaving them in the porch until a neighbour saw her and let her in so she shouted at my mum for 'disturbing' the neighbours). Basically - things were a little uncomfortable between his wife and my mum. I think it was just her old age but my mum thinks it's that she married my dad who's foreign and they're a little old-people-racist.

Because of that awkwardness, I saw them quite a bit growing up but that cut down a lot when I was an older kid and I only saw them once every couple years. Now I'm almost 20 and the last time I saw them was a year or so ago.

Thing is, I got along really well with him and he felt like the closest thing to a grandad that I had (all my grandparents died before I was 2 and I have no other family in the country because my mum was an only child with no relatives). Especially as I got older because he used to be an engineer and I'm a physicist so we had a lot to talk about and I loved his humour - he was basically the only old person my family visited that I actually wanted to see haha

I just feel like absolute shit though because I've been pushing my mum to take us to see them for a couple months now (they live close enough to walk but I could only go with her because he was blind so his wife was always there and we didn't have any conversation so even calling her up to thank her for cards and that was painfully awkward) and she kept putting it off and kept making excuses just like she has been doing for years now. I lost someone else two years ago and regretted not sending a letter I'd been thinking of writing for a couple weeks so I promised myself I'd never let that kind of thing happen again.

I know my mum has reasons and probably feels guilty but I'm finding it difficult not to feel angry at her for me not spending some time with him before he died. I've always felt so lost without grandparents so him being my one way to have something like that relationship and losing him feels awful.

Have any of you ever been in a situation like this? How did you deal with it? Even if you've not personally been through it, what are your thoughts?