Won't go to sleep without help

My 18 week old will not go to sleep on her own. She cries (screams) until I calm her down by bouncing and patting her. Then when I put her in her crib she starts again. She has to be asleep enough in my arms in order to get her down to sleep in the crib. I've tried patting her in her crib for like 20 min, but that still doesn't work. Same for in the car, she just has a meltdown. Any advice?