My tongue-in-cheek answer is you need to find a group ride with a couple of fast 50-60 year olds and get your teeth kicked in a couple of times. It will be both humbling and inspiring. There are a couple of guys I have ridden with that have given me hope that I can be bringing the hurt to the 20-30 year olds well into my 60s if I remain consistent.
I’ve been riding since 2004, mostly unstructured. Last year at 41 I was the strongest (FTP/TTE at various levels) and lightest that I had ever been. I started using TR last August to add structure and got some decent FTP gains just from that. A lot of this year (at 42) my FTP has remained roughly the same, but I have increased TTE at various power levels and have set PRs for avg power and avg w/kg for 3.5min, 5min, 20min, and out to 4 hrs.
My view of cycling is it is a long game. It isn’t about weeks or months or quarters. It is about years and decades of consistency. My FTP plateaued for years at a time, but now if I can bust through the current plateau and increase my FTP by even 1-2% I will be super happy with a new PR and consider a year of hard work worthwhile.
If you had been riding since you were 10 and followed a structured plan during much of that time, sure there might not be much room to grow at 40. But that is a hypothetical comparison. You don’t know where you could have been at 30 if you started at 10 and trained hard for 20 years. Comparing yourself to that hypothetical is only going to demoralize you. Compare yourself to you 1 year ago - 16-17% increase in FTP is HUGE! Don’t forget that.