Struggling with the doctrine of hell

The more I really think about hell and what its like I can't imagine how our God of infinite mercy would allow anyone to go there. I realize He grants us free will but if we were faced with the reality of heaven or hell no one would choose the latter. I can imagine a place of temporary suffering, like for example on Earth we have jails, but infinite painful suffering? I realize that sinning against an infinite God requires infinite punishment but it just seems quite excessive. Even for the worst of criminals they aren't tortured here on Earth. And most people in hell will be pretty normal people I think, that just rejected Jesus. Another question I've been having is why can't someone repent in hell? Say they truly want to follow God and be with Him in heaven, why can't someone repent and turn from their ways after they die? Does anyone have wisdom/advice on how to approach this topic that I'm struggling to come to terms with? To be honest it makes me feel resentful towards my creator and I do worry quite a bit about hell, even though I am saved. Thank you