"Hopeless" was a bad translation from a word in my native language
I mean more like "should not be relied upon".
I've learned self-defense from an experienced police officer. The first thing he taught me was that you should never rely on it. If someone asks for you for money, give the money.
If you're walking home late at night and come across a gang of creepy people hanging around there, don't stick your head up to be intimidating or don't keep your head low to look non-threatening. Turn around, walk to a brightly lit area, even sleep in a public area if you definitely have other choice than crossing that dangerous area.
If someone tries to attack you, kidnap you, or rape you, then use the self-defense techniques you've learned. Use it only to neutralize them, not to get revenge or intimidate them. And don't use fancy stuff like judo throws, roundhouse kicks, or groin kicks - gouge the eyes or punch them in the throat. Only use self-defense if you're that desperate. Assume your odds are around 30%, especially if you're a light female and facing against one or more opponents who has anticipated combat.
Most martial arts are scams. They are not applied to serious combat situations. Even MMA rewards things like groundfighting and wrestling, which don't work so well when you're facing someone armed with a knife, who's willing to bite your face off.
Most of us will learn self-defense for a few weekends a month at best. There are people, who out of raw insanity, can take a punch straight to the face and not feel it until hours later. There are criminals who spend years in prison learning how to fight. There are meatheads and bikers who have been fighting since their early teens. There are people who grew up in violent communities since birth and were forced to learn to survive. There's no way someone who's been learning a technique part-time can beat people who have actual experience.
If you're facing an abusive relationship, sorry, but fighting is not the answer. There are better solutions, political, social, or legal. Or just plain moving away even if it costs you a lot. If a guy throws boiling oil in your face, a karate black belt isn't going to save you. If you break a guy's arm in self-defense while he's trying to hit you, be prepared to face an annoying legal battle, even though you did it in self-defense. If you fight back against an experienced fighter, you'll piss them off and encourage very bad things.
There is the possibility that an abusive boyfriend/spouse might be less experienced in fighting. But I wouldn't rely on it. Everyone should fight back against an abuser, but physical defense is an absolute last resort.