1. You have to understand you are going to make painful mistakes as new investor. It has nothing to do with lack of knowledge... its plain greed & fear. Do avoid investing more than 50% of your asset before you get your first taste of pain... only after which you'll learn where your risk tolerance is at.
2. Holding your investment through crisis is a subjective question. Nobody can time market correctly all the time, best advice is usually to hold onto it until it recovers. However some people simply cant sleep looking at losses everyday... so an alternative choice is to cash out 50% of it first... having some cash always opens more opportunity.
3. If you are a trader, you should have price target for selling of investment (both up and down). Once you made up your mind you should execute it diligently with no emotions attached.
Whats most important is to have the right mentality and habit.