Finally, I'm blue! I'm looking for advice to keep improving.

I can solve consistently problems A and B, and most of the time C. However, I usually have little time left to address problem D.

I've been working on more difficult problems and algorithms, but I never get to try them in contests as I'm not able to solve A, B, C fast enough. What do you think is a good strategy to improve? should I try to be more fast on the first problems, or should I force myself to work on problem D in the contests?