I couldn't agree more about Appen. This from my investment journal:

01/09/2021 Sold all of APX, after uninspiring FY21 results. I’m no longer convinced that they have market leadership, because the market is changing rapidly. Loss from my investment was -46%. Ouch!

i guess there is some consolation from the fact I sold at $10.50, but I was slow to react to changing market dynamics.

I made another mistake when I lost conviction in NFLX. Again from my journal:

20/07/18 Sold half of my position in NFLX for 365. Concerned that growth rate is not sustainable, and that rate of increase in costs (from producing new content) is exceeding rate of increase in revenue. Also high debt. On balance still a market darling with lots of growth expectations. 

25/10/2022 Sold all remaining shares in NFLX for 290, taking advantage of pop in share price after they announced decent Q3 results. Despite new ad supported plan, I think the company is going to struggle to show meaningful profit growth in the future. The streaming market is getting increasingly competitive, other players like Disney have deeper pockets, and the cost of financing their $7b budget for new content is going to increase appreciably with climbing interest rates.

With shares currently at 444, the market clearly doesn't agree with my assessment.

Despite these examples of selling too late (APX) and too soon (NFLX) I remain pretty happy with my investment process, and with my determination to only discard a company when I believe the future opportunities for profitable growth are no longer there.