ELI5: Why can’t I continue to contribute to an HSA after changing to a Copay plan?

I’d like to consider myself at least a partially functional adult. I’ve been in the workforce for over a decade; I pay rent, keep a dog alive (and thriving), change my tires – I even have a financial advisor. I’ve just paid off a car note and am saving to buy a house. All very adult things. And I’ve been dealing with the American health insurance scam for my entire life. Not gonna pretend I understand any of it – for the most part I just go to any appointments I have, and for the last ten years or so I’ve been using a copay plan because that’s what made sense to me. This last year, though, I switched to an HDHP specifically to get access to an HSA, because there are some medical things that I want to be more on top of (mental health, medication, etc.) and I wanted to see how it would work. And actually, I love having an HSA, because it means that no matter what else is going on in my life I know that I have money for my health needs. But I’m not a huge fan of the high deductible (my company’s plan has an $8,000 deductible that that is, frankly, obscene – why even have insurance, if I’m paying $8,000 out of pocket to even use it?), so now that open enrollment has come back around I’m looking at switching back. I know that I’ll still have access to the funds I’ve already accrued on my HSA, but in an effort to be proactive I’d like to keep contributing to that to build myself a safety net – even if that means doing so after taxes. But all the research I’ve done says that there are actually penalties for that? Which I don’t understand. The HSA is my money, and the money I want to contribute is also mine – I literally just want to set money aside that I can only access for medical needs, why can’t I continue to do that? Alternately, is there a way I can do that?

Please explain this to me like I’m a very small child, because I’m about to fall into a coma with this alphabet soup crap 😤😅