People are probably going to give you a lot more detailed advice but in my experience 99% of the time when people say “false twin flame” they’re really talking about a “karmic twin flame” which is really just a karmic connection.
They can feel big and important when you don’t know any better because they’re way more intense than any “normal” connection you have with people in the 3D, but, in hindsight, they’re very easy to spot.
The main difference is that a false twin flame will be intense - but not in a way that grows or builds either of you. They’re there for you to learn something from (or for your twin to learn something from) and they leave.
It’s not possible for them to have a happy or long-term relationship because that’s not their function.