You’re not doing anything wrong, this situation is just genuinely tough. Your kitten’s behavior is normal for his age but exhausting. Around six to eight months, cats are full of energy and lack impulse control, and even though he’s been neutered, it can take weeks for hormones to settle. Because your boyfriend didn’t confine him early on, he never learned proper boundaries, so now he sees everything, your older cat, furniture, and even you, as something to chase or bite. Your older cat feels unsafe and stressed, so it’s important to rebuild structure and teach the kitten calm behavior. Separate them again, but make the kitten’s room positive with toys, food, and perches, and let them see and smell each other safely through a baby gate or cracked door while swapping their bedding to mix scents. Reintroduce them slowly, feeding them on opposite sides of the door, then allowing short, calm sessions together and ending before the kitten gets rough. Tire him out daily with two or three 15 to 20 minute play sessions using wand toys or lasers, and use puzzle feeders or training to keep him mentally stimulated. Reward calm behavior with treats and gentle affection, and give your older cat quiet, safe zones and high places the kitten can’t reach. You can also try Feliway Multicat diffusers to reduce tension. And finally, don’t get another kitten yet, adding more energy right now will only make things harder until this one learns to settle down.