my dreams of late have all been weird, and never about the one thing for very long.
In previous times I might've had dreams which go on and on and sometimes carry over to different nights but lately, short and random,
I've had a few dreams about my late mother recently which always upset me a little. She died 19 years ago. The dreams are nice and it does feel like I am seeing her in person again however it still takes me that split second to realize that she is not here anymore when I wake up.
Your subconscious may be looking at various elements of your psyche for which your mother (and your relationship) were very important factors in your development; emotions and thought processes that were put on hold since losing her, i.e., certain concepts of parental love, assurance or dependence and whatever else (it wouldn't want to guess as these vary widely from person to person).
Your subconscious is now reviewing and reminding itself of these memories and feelings like the emotional equivalent of and old but extremely important smell, taste or sound. Perhaps for the sake of development, or perhaps for the sake of comparison to your current emotional living environment (There could be many reasons).
It's my guess that the melancholy in waking after these dreams and being reminded of loss is a side effect and not the main point and thus shouldn't be dwelled on. Rather you're better off embracing the dreams while they're occurring but regard them stoically upon waking and tell yourself that's just your subconscious doing it's thing and keeping your emotional equilibruim fine and healthy.