I think there is also a selection bias that the people who post on this forum are mostly active investors who have young portfolios and are seeing the illusion of a good return.
Fred93 has done the hard lifting of pointing out the real selection biases involved in the issue you're experiencing, but, as far as
"forum-demographic", many of us have been around this industry since ~2006 - and some of us since before ZOPA UK's debut...
It's not the first time we've seen someone discover adverse selection, the effect of new loans (or prepays) on return metric, etc.
I will note, as well, that you mentioned "tax consequences" as a factor in whatever return calculation you may have been using
to model your portfolio's performance. Investing in this asset class outside of a tax-efficient vehicle (IRA/etc) is "not optimal"...
That was
my clue that it was
you who is the rookie - rather than your initial claim that the real problem belonged to all of us.

There is an EOY/holiday effect that usually affects performance during this time of year - but that is a higher order "problem".
Better luck to you (primarily in the sense that you reduce your dependence upon "luck")!