Why is my tabby cat so needy?

I have this 5 month old tabby cat and she is extremely needy. Whenever I lay down, she sits on my chest. She is always kneading on me. She always wants to play and when we leave she gives this heartbreaking little meow. I feel bad for her, but I also want her to be independent. We have to older cats in the house, so she can play with those, but the only way she knows how to play is attacking them. My older cat is getting to the point that he is vomiting from the stress. How can I get Willow to be less needy?