Four main branches of western philosophy:
1. metaphysics
(a) ontology (what exists/is real and what doesn't/isn't)
2. epistemology
3. logic
4. axiology (value theory)
(a) moral philosophy
(b) aesthetics

If happiness of the past is a feeling of satisfaction of satisfaction, contentment, pride and serenity, no wonder unhappiness of the past is about a sense of regret, "if onlys", nostalgia, wanting to get back. Not to mention bitterness, revenge, etc.