From my own learnings on a horse very similar sounding to your second - look into PSSM2. Cross-cantering to the left was my mare’s only notable symptom…until increased work spiraled us downward into spookiness, bracing, blowing through my hand / strong on the bit, heavy on a single rein, etc as her symptoms became more painful. Added Vit E, magnesium, and a lot of protein to her diet and she’s lovely! Took 2 years of ruling out EVERYTHING else to get there though. It’s a (relatively) cheap thing to try and see if it makes a difference?
OP - as you can see above, I’ve been there too. Fortunately for me, I don’t enjoy showing so my ambitions were clinics and to enjoy my horse with varied recreational fun (hunter paces, foxhunting, XC schooling, dressage lessons, trail riding, etc). Even when she was NQR or straight up lame for proper work, I could work on foundation-building for those things with in-hand work, desensitization, bonding, hand-walking in the trails, etc. So while it was incredibly frustrating, I could still enjoy her and feel like progress was being made (I bought her as a 3 year old, problems cropped up as a 4 year, mostly solved by 7).
I think for me a choice point came in looking for a horse of do I want something to enjoy the journey with,(but the destination is unknown) or do I have a specific destination to get to. I realized that with a demanding corporate job, plans to start a family, limited [horse-dedicated] funds, and only moderate talent…embracing the journey (even if that meant ultimately a pasture puff) without a destination was going to best mitigate risk of heartbreak. So I focused my search on personality and brain, with no obvious barriers to “goals” of 3rd level dressage and low-level jumping (obvious being the key word here). I love my mare, she’s a hoot even when I can’t ride, and it made the 2+ years of NQR mystery-solving a lot more enjoyable even as we frustratingly experienced no evident progress on riding “goals”.
There’s no right or wrong answer to the question, but it’s probably a question to reflect on: Do you enjoy the horse enough to not get anywhere competitively or is it time to try again on finding a horse that will get you there? It’s a spectrum rather than a binary, but it can help inform if or when you arrive at the "throw in the towel’ milestone.