From my experience you are going in the right direction as you said, you can handle a module. Confidence will come with the time. Also the more you know about the requirement of the system, better you will be able to design the system with little or no help. Certification, though it might give you some confidence but can not teach you how to design that you have to learn by yourself. In reality its almost impossible to design the system perfectly in one go, infact you have several iterations to improve your design and coding. Remember, never design for future, but keep your system flexible enough to adapt change. Its a difficult combination, that you will learn from your experience. Also a tip - if you like someone's design just ask them how they started and did that.
My suggesiton would be concentrate on the requirement. See other project requirement / design document in your company and learn from your's and other's experience.
Once you have gained some confidence, go for SCEA. 2nd part in the exam is bit tough if you are new to desin but it will polish your skills better.
Hope it helps