I'm curious about hardcore fetish, but worried about safety…

When someone has you immobilized, or gagged, or they're using a potentially dangerous sex toy on you, safety is certainly a concern. When engaging in BDSM activities, make sure your top has experience. He should be able to explain to you things that he watches out for and tries to avoid. He should tell you how to communicate that the scene is going badly for you and you need it to end. Even if you're gagged and bound there should be a way to communicate that fact to him. Trust and talking ahead of time are crucial for serious, hardcore fetish.

A scene went badly, what do I do?

There are many ways a scene can go badly. If you didn't communicate properly to the top, then you should learn the lesson and communicate better next time. If the top made a mistake that caused you to not enjoy yourself, then you may want to give him another chance - especially if you liked him otherwise. However, it's also possible that you were just flat out raped - that what happened to you was intentional and deliberate. In this case go to the authorities and report it as a crime. There's no need to apologize for liking S/M. You should be able to indulge your fantasies without getting raped.