Is memory biased towards positive or negative memories?
The short answer is: it depends on age. For younger adults, negative memories last longer than positive memories. The opposite is true for older adults (above 60 years old).

This paper is a good review of some of the above effects: Choice-supportive bias, Mood congruent memory bias, Positivity effect, and Negativity bias. It suggests that negative bias only exists among younger adults, while positive memories last longer than negative memories among older adults. The paper also provided several explanations. One of them is that older adults tend to exhibit auto-activation of emotional regulation ability, thus cognitive control comes into play during the memory recall process.

Mather, M. (2006). Why memories may become more positive as people age. Memory and emotion: Interdisciplinary perspectives, 135-158.