I am a short cyclist expert. A store owner for over 12 experience helping with sizing/ riding styles. With a 51.5 cm top tube the frame should be plenty short. This is likely about your riding style. With modern road bikes, a lot of (road) cyclists have adopted the style where they ride all of the time on their brake lever hoods. I have some cyclists who basically can't/ wont take their hands off the security of being within close range/ right on top of their brake hoods. I suggest that you spend a lot of time on the tops and switch your hand position to the hoods for shifting, braking and when getting off of the saddle (while climbing etc). I think you will find that you actually need a longer stem.