Investing Dilemmas

The basic dilemmas before buying a stock are:

Will it go lower or should I buy it now?

The classic dilemmas after buying a stock are:

If you are making profit, will it go higher or should I sell now?

If you are making loss, is this a dud stock and should I sell out? Else, will it go lower or should I average it now?

Other dilemmas:

I bought this stock for a long term basis, but it has shot up so fast so soon. Should I sell it now or keep it for long term?

At what price should I average it? At 5% below previous buy, or 10% or 20% or some other number? I obviously cannot keep averaging it at a lower price as it would affect portfolio and sector concentration.
What amount should I keep in cash waiting for opportunities to buy that may arise in the market?