Jax,
Everyone struggles with this aspect of trading. You just need some simple money management rules that forbid you from giving so much back but allow you to continue to go after more opportunities.
Since you are already aware of how much you are up while you trade, you need to keep tabs on how much you are giving back and have the discipline to stop. For me, I more often than not finish near my high of the day. However, I have rough patches just like everyone else where I need to make adjustments as the market changes. During these times, I give myself a down limit before I'm done.
So you are up $700 and blood is rushing through your veins out of excitement... that's good. So now, you need to tell yourself that you will risk $100 before you have to take a break from trading (and your break must involve you getting up and leaving the market for at least 30 minutes). Once you come back, you have credit for another $100 to take advantage of more opportunities. If you lose that, then you are done and you are $500 up for the day.
You can use any limit you like for how much you will give back. I figure $100 is good because that is a point on 1 contract for the ER2.
The bigger issue is whether or not you are too attached to your P&L. With myself and most guys I have worked with, this is always the case. It is the old "man, I was just up $700 and I lost $100. I need to get it back." Those last few words are what get you in trouble. Don't trade to make a specific amount and don't trade to make money BACK. Just trade to take advantage of the opportunities you are given by the market. PERIOD. Trust me.... if you ever even consider that you need to take a trade to make money back or to make a certain $ value, then you are setting yourself up for a rough road. 2 out of 10 times, you might get what you want but you will pay it back in the long run. This has been my constantly stubborn experience.
Strong attachment to the results of your trading in monetary figures is the demon that I think gets most people out of balance. Consider what you were thinking the whole time you were giving your money back and you will probably come to the conclusions I am outlining here.
Best wishes and good luck.