As previously stated the two are completely different skills. It is extremely unlikely that you will be good at both as they are almost complete opposites in the skills and talent required.
Perhaps the reason you are having trouble learning development is because you are more the type of person who is good at design. If you do try design and find you are good at it then concentrate on learning more design skills and forget development.
I have never come across anyone who can do both at anything beyond beginner level. If you find you can move beyond that in one area then you almost certainly will not have the ability to do the same with the other.