Supposing I have exposed dentin due to aggressive brushing, do I still brush the dentin part of my teeth?

Brush it all. You need to clean the whole tooth to prevent gum'line cavities and gum problems. Dentin is softer than enamel and more likely to decay. Use a soft brush (especially if you are aggressive); there are many inexpensive electric brushes that are good. Many times recession can be caused by clenching or ginding your teeth while you're sleeping and unaware...