Can't really expect 'em to live forever, ya know. It's not your fault that every living thing has an expiration date. You've taken care of 'em throughout and that's all that matters. Life's a journey. Just gotta see it through to the end, the good, the bad and the ugly.

Well, it is our responsibility as pet owners to make it as long as fruitful of a lifr as possible

In as much as any creature is able to comprehend, all life can tell the difference between ones who makes an active effort to comfort and protect, versus those who just don't care. That the effort was successful or not is, in a way, less important than the effort itself. You did the best that you could to care for your bettas that you were able, and their spirits, either in this world or the next will speak of that care and pass it back around to you when you need it. Even when exiting this realm, they leave behind a snapshot of your soul. Be it one of kindness and caring, or cruelty and hatred, there is nothing that speaks more truth of your soul than how you treat a pet. Whether that comforts you, or concerns you, it can at least guide you going forward.

It sucks, but sometimes things happen we just can't control. I'm sure you love your fish and that you want them to live, but things happen and sometimes we just can't control it. That being said I'm sorry that this is happening.