Thank you so much Badger, Lisa and MartianDestiny for your input.
I know you're right.
I think I'm beating myself up so badly because at the time of adopting Jasper I became very friendly with his handler (who had lurchers of his own) - who made it quite clear when I returned him that he was disappointed in me and thought I hadn't tried hard enough. That was despite having written to him before returning the dog, explaining in details my reasons for doing so - and completely accepting responsibility for them.
The guilt isn't appeased by the fact that Jasper's still waiting in the shelter for someone to take him.

Ironically, at that time I wasn't riding much at all, so the bike wasn't an issue then - but my partner worried that we'd not be able to go for longer rides because I only managed to get Jasper up to about 3 hours being left (admittedly from a starting point of 3 seconds before he'd bark constantly and pee in his crate!).

I want so much to be a responsible dog owner, but guess that I need to just focus on actually enjoying dog ownership, rather than worrying about how I might be failing. I'm sure that Jasper was much smarter than me and picked up on that a lot of the time!

I'm thinking about a few breeds - non-shedding (partner has asthma) - which kinda leads me along the poodle/terrier route. I know - smart and bossy! I want a big dog. I previously had an Airedale, whom I adored.
TBH though the idea of a thoroughbred is a little scary - puppy farming and bad breeders. I do like the idea of a mixed breed, but don't really have any fixed ideas - other than no collies or spaniels as that would definitely be a potential problem with being left.
I'm just taking my time to get my head around the prospect of having another dog again to begin with!
I did think of volunteering with the local dog shelter (not the one I got Jasper from), but decided it might be quite a dangerous move!

Anyway, now I'm rambling again. Thank you for putting things into perspective for me. And apologies again Badger for hijacking your thread.
I hope you find the little guy of your dreams. It's a shame we don't live around the corner from one another and could co-dogsit!

I think I'm going to spend the next month or so having an "imaginary doglet" to see how well it would potentially fit into my life. When I start taking it for walks, I'll know I've probably decided!