I Promise this Isn’t a Self-Help Blog
I read this Arthur brooks essay in 2019 and enjoyed it — my main takeaway was, think about a professional/life arc that curves toward coaching and teaching in the back half … https://www.theatlantic.com/magazine/archive/2019/07/work-peak-professional-decline/590650/
I'm not sure I have much in the way of recommendations, but gobeithio eich bod yn cael hwyl yng nghymru :D
I guess this is in the self-help genre, but something else you could look into are books/etc about grief. There are a lot of terrible ones out there but I read It's OK That You're Not OK by Megan Devine and found it helpful for dealing with grief, but it also touches on life/meaning (because grief will be an inevitable part of life for most people).
For this project, I would recommend a memoir called It's Okay to Not Be Okay. A memoir might sound like an odd suggestion for your project, but as a therapist the author had an interesting take on the self-help genre. Some of the stories about her clients might be helpful in thinking about the subject.