This is true to every entertainment industry though, such as singers and their albums, and artists and their artworks
I'm not too familiar with other industries, but when it comes to publishing, usually you'll sign off publishing/exploitation rights, which isn't the same as copyright per se, which is in essence proof that you are the author of that work. If the publisher breaches the contract in any way, i.e not paying or publishing by the deadline, all publishing rights revert to you. That's why most authors get agents before signing contracts, although I know things work differently online, and agents are another can of worms.
An interesting case of an author who sold her copyright was L. J. Smith, author of the Vampire Diaries. She doesn't own any of the characters she created. Harper Collins brought in another author to continue her series when they fired her, and she started writing "fan fiction" of her own work, which her fans recognise as the true sequels of her novels. So that's actually a loophole you could use, but I get that you don't want to risk legal action.
That fucking sucks though. I'm sorry you lost rights to your work.