Are people with overactive imaginations more likely to have/get psychiatric illnesses?

Not unless you become a danger to yourself and those around you. Ever heard of Stephen King? They say he's a tortured soul, he has an overactive imagination like you, that others would find disturbing. But look at the creativity it portrays in his writings. :D Be proud of who you are, and use your talents. If you feel that you are a danger to yourself and cannot handle this anymore, go get help. But if you don't fear it anymore, if will probably get better on its own.