Check how your body rotates, mostly if your hips or your arms start the down swing.  My guess you are starting with your arms. 

Best thing I can recommend is the Swingyde training aide. Look it up online. Won't set you back too much. It's a simple and easy use device that you attach to the grip of your club and helps you get into the correct position in your back swing. If you can get into the right position at the top the downswing is a piece of cake. Just drop it down into the slot and your good to go.

i used to have the same problem untill i took lessons. Witch is my number one choice for advice is see your local pga professional. but something my pro used to help me was.  Youtube Jim mcleans 8 step swing. Use that as a base ground for developing a new swing plane. For you really focus on the first part of the down swing. my Pro said its like hitting a nail with a hamer. i found that if i get the feeling that im pulling the clubs as i rotate from the top i seem to bring the club more on line with the natural swing arc your looking for. give it a try let me know what you think.