The platform already has the features to do what you've requested. On any question, tag or user page you would like to bookmark, find and press this yellow star button:
Doing so will add it to your favourites list, which you may also access though the grey button on your profile page that says 'My favourites'. Note that you have a separate favourites list for each section.
Similarly, you can use your updates area to receive notifications when someone responds to your post, including when the post is old. You can also use your activity lists to locate any post you made in the past.