I've been through this scenario many times, something either looks weak (drifting or levelled out) and then the full blown reversal or steam down comes late and just doesn't retrace and I'm sitting there watching it as it goes lower and lower into the red. Contrary to this a favourite can look like an "obvious steamer", (the previous favourite just won which adds to my opinion) so I back it and then a couple of minutes to post it bounces like a clown's bouncy ball and goes up and up and up
I think the key is to first work on getting a good entry, learn how the market moves and don't enter somewhere where your stoploss is a red bus away which will either throw you out or leave you with a big loss. You then have to get used to cutting the loss short if you are wrong and move on. It takes a very long time to accept that your opinion is wrong (in fact even some good traders only have 50% strike rate and spend half the day scratching). I was told this 2 years ago and still struggle to think in probabilities and the long term sometimes, we just naturally always want to be right. Trading is very emotionally testing even to patient and mathematically thinking non-gamblers