Quote from eagle:
It's all about the relation between a trade size and your account size as a whole.
If you're too nervous with 16 cars it does indicate that your current psychology is not yet to support this trade size comparing to your account size. At the moment you have built up your account bigger enough then the 16 cars will become as 1 or 2 cars and then the 64 or 128 cars will become a very stressful size and so on.
Eagle hit the nail on the head. He is exactly right.
I will further explain from another perspective, albeit making a few assumptions along the way.
How many times have we been able to hold a contract for 30 points after being profitable right from the point of entry? Close to never most likely. You find that you get out after being profitable for a few points because that is your expectation. Simply to make a profit.
On the other hand, how many times have we been able to hold a contract for 30 points after being down 30 points in order to get to break even? This has been done probably more times than we would like . The difference is in the mindset to breakeven of course. Ultimately, you instill in yourself that you are looking for 30 points. Simply making a few points is inconsequential you come to realize. And thus, are able to hold for those entire 30 points should it materialize.
So what do you do? It is difficult, but simple. Your mindset should be to make 30 points on each and every trade. That is what you should be aiming for psychologically. Because most traders have experienced this before, it is something that most traders can do, but something that few actually do.
Stop loss, money management, setups - those are other aspects which can also be used to address this issue. But your question was one of mindset, so I tried to answer within those terms.
Hope this helps...

