I have had a bad time selling also. Seems to me that the state of the markets has a lot to do with it.
When the markets were bullish, I had no problem selling but buying was. Now that all stocks are undervalued and can be beat up upon at any time I do not consider it hard to buy at a good price. If I buy and the sp is dropped some more, I buy more.
However having bought at an average low price I find it hard to determine a good selling price. This is due to a stock that is priced well below it's value, I almost always think that the market is fairly sensible and will tend to give near full value to a stock with recent good news and being the attention of investors - Wrong.
Almost always the sp stops appreciating before my expected sell price.
I have been trading stocks since 1986 and by means of the internet since late 90s but with all that time I have still a lot to learn.
So if you are having similar problems, do not fret too much, you have plenty of company.