The article in the link below by cello Professor Miranda Wilson, gives useful practice tips on how to carry out the sudden dynamic changes found in Beethoven’s compositions.
Other than Beethoven, Bach’s compositions also have sudden dynamic changes in parts that echo from forte to sudden piano with no diminuendo or crescendo.
I often practice these sudden changes by stopping after the end of the dynamic, just before the sudden comparably soft or loud dynamic change.
For example, complete the forte portion completely without any hint of diminuendo, give a slight halt to prepare for the next piano section. This is an additional way to this author’s idea of practising out a sudden drop or increment of dynamics. You can do the same with a piano portion, practice without a hint of crescendo, stop before the forte section then prepare and continue with the forte.
After a certain while of practicing in this fashion, you will be able to execute your the sudden dynamic changes more effectively for the listener and your ear and muscles get used to not committing any unnecessary diminuendo or crescendo which is not intended by the composer, the sections will be kept distinctly and the character of the piece will be executed well. Do try this method of practice and let me know if it works for you too! This method is not confined to violinists but all sorts of other instrumentalist should try it too.