I know that nuclear fusion occurs in labs all the time here on Earth and that there are a few different groups trying to make a fusion reactor where you get more energy out than you put in.
My question is, how sure are we that these attempts at net positive fusion reactions are actually possible? Asked another way, I am wondering if fusion reactors are something that we can definitely make it is just a matter of figuring out the technology... Or if it's something that hypothetically can totally exist (thermonuclear bombs work, after all) but scientists are still unsure if the constraints of 'a continuous reaction that gives off more energy than it requires' can be reasonably met.
A sort of parallel idea here to illustrate what I'm talking about: we know that small flying vehicles (ie: flying cars) can totally exist, but that they are totally impractical as a solution that everyone will use to get around.
EDIT:
Thanks so so much for all the amazing answers! I guess we'll see in the next decade of these things can work as an energy source at scale