Losing your mojo

The whole thing of comparing my previous riding AND other people's current riding to what I'm currently doing is essential to spurring me on though!

It shouldn't be essential. Especially coming back from what you've gone through. Finding a way to spur yourself on without hating yourself will take you to the same destination in a better headspace. You're going to get there because you want to get there (really really want to get there), and these things take a lot of time so don't beat yourself up about how long the journey is.

I've been riding ('proper' riding, not commuting) a lot since the beginning of January in an attempt to undo 18 months of off-the-bike. It has taken a huge investment of time, energy and potential (sometimes actual) misery to get to the point that I can ride without hating how rubbish I am at it, for it to be comfortable and, soon (I hope), easy.

So. Smile, breathe, ride your bike. In a month or two you'll wonder what the problem is as your blitz around your favourite loops at the speed you used to.

attempting to match a few people I know on roughly the same distance and/or speed over that distance - it's one of the few things I have at the moment that might give me some vague sense of being back to normal. I don't really expect anyone to get exactly what I mean, but basically I can't just accept a 40 mile ride in the sun as being a good lark, and "well wasn't that a nice day out" etc. If I realize I could have done another 10 miles on the end of a ride after the fact, I'm going to feel shit about it.

Every ride in January I was comparing my average with my possibly-rose-tinted-memory of how I used to ride.
This is fruitless.
Doing this doesn't make you faster. It makes you feel worse about being unable to overcome barriers that are impossible to overcome. You don't gain a mph on your average by knowing you used to be able to do it and hating yourself, you get it by riding, riding and riding some more. And the way to do that is to not hate riding your bike.

I have to push myself and push myself, and say fuck you to the weather, fuck you to the pain, fuck you to being delirious from exhaustion all the time, and use the very finite and indeterminable amount of time there may/may not be before I potentially get ill again like last year, to get myself as strong as possible for a potential 2nd round of what nearly killed me last year.

Yes. This bit is acceptable. Having some fight is good. Desire to get back on top is good.

It's sucking most of the enjoyment out of riding, but I have to plough on.

Necessity kills fun, but can be unavoidable. Good luck.