knowing when to sell

As an investor i find deciding when to sell a stk much harder than deciding at when to buy it. I also find making a decision re which price to sell at...... much much harder than at what price to buy at. My tendency these past 4 months has been to sell too early. As a result i miss out on getting that extra 10-15% from a trade. I consider myself a medium term investor so ask myself why am i selling at all?
My decison making is driven largely because i am finding it very difficult to make sense of the mkt and get a handle on what direction it will take. Doubt i am alone as the graph shows wild swings since the November lows. So by default i have become a sort of monthly trader.
I am also driven by fear. huge lossses last year followed by series of successful trades these past 4 months has made me ensure i will not exposure myself to bankruptcy again. Hence, i would rather sell NMS at 49c and bank a 18c/share profit..... than hang-on for 54c (which it reached today).
As we get nearer the august results i lift my buy-back-in price.I expect to pay a bit more for buying nearer a dividend and in the absence of bad news. Hence, i will buy back into NMS should it fall to 40c. I will buyback into UGL at around 8.oo as opposed to the 7.64c average previously.
I am far from convinced overseas mkts have bottomed and have reduced my portfolio substantially these past 10days. But am def not prepared to sell BLY at 19c or VMG at 27c for example when i feel there is a good probability of strong upside in SP.
Making a decision often depends on the circumstance faced by the holder. Hence, everybody's situation/risk profile is different. But i still struggle to know when to exit a position.