Have you guys ever had the fear that other people don't have mental states and are just zombies in a way? This makes me feel very lonely.
Yes, but why would it be so? They have the same hardware as you do, the brain, and all evidence points to the brain producing consciousness, consciousness being entirely dependent on the brain organ. Therefore, why would they not be conscious just like you, given they have the same consciousness-producing organ?
It's a fun thing to ponder, but there's absolutely no reason to believe solipsism would be true.
Also what is there to fear? Given that they display the full spectrum of behaviors as you do and at least "fake" these mental states, what does it matter even if they were somehow fundamentally different from yours? I don't see why it would really matter as it wouldn't change anything about anything. The world keeps rotating and people keep functioning in full capacity, regardless of whether their emotions, sense of pain, etc are identical to yours or exist at all.
Eventually we'll probably have AIs that are on humans' level in terms of intelligence (and beyond). Does it matter that they don't feel the same way? Well, I can't answer that fully right now as it involves complex moral considerations, but you get the point.
The bottom line is that it doesn't matter because it changes nothing. Even if it were confirmed to be true somehow (impossible scenario), you would still feel a deep aversion to hurting or killing another human being, even with the knowledge that they feel no pain the same way you do. But it's not true, so you can just forget about it.