Why is it that when I remember good times in the past, I get an overwhelming feeling of sadness rather than happiness?

Context. "golden days" are almost my definition in the past. We tend to forget the not-so-great aspects and idealize the good parts. As a result, current circumstances rarely measure up to such memories. If one adds to that a present that is full of stress or that is far less ideal than what you'd looked forward to way back when, then past memories may evoke a sense of loss rather than gratitude.
DEPRESSION. Get an evaluation from a psychiatrist to help identify what depression means to you and whatever else may be bothering you. Consider recommendations for medication if any are given. Get a referral for a reputable psychotherapist to speak with. Meds and therapy combined are most optimal; no just one or the other.