Common Mistakes That a Retail Investor Makes

1. Trying to time the market.

Trying to catch the top & bottom

Some think they can buy at the bottom and sell at top. It's very difficult.

2. It will rise again

After buying a stock they justify themselves by saying that its a good company and that they will come back.

3. Can't go more lower than this - Trying to catch the falling knife

It sure can. Again you are trying to time the market.

4. It won't go above than that

It sure can. Again you are trying to time the market.

5. Averaging 

Buying same stock again to average when they should actually exit.

6. Following the market gurus

Listening the news and then just investing on the basis of that. It works sometimes but there are many factors that affects the market. So, we can't really follow that.

I made some mistakes by listening to them and not using a stop loss.

7. Thinking on becoming a millionaire by Penny stocks

The small penny stocks can be easily controlled. So, be cautious and use stop loss.
Read http://bleedingmarkets.blogspot.in/2016/02/the-monkey-story-on-how-stock-markets.html

So, How can a retail investor trade and invest if there are so many problems?

There are 2 approaches.
1.Fundamental - In which you read the balance sheet, read quarterly results and all that sorts of thing.
2. Technical - On the basis of charts, following trends.