You're picking doesn't really suck- I was just getting your attention! However, many people struggle with the transition from strumming all the strings to picking single strings. The first thing to understand is they are very different actions.
Strumming is an arm movement from the elbow, your wrist should be pretty still and close to the guitar.
Picking is a wrist movement. Your wrist should be resting on the guitar above or on the strings.