NDX has Strike prices mostly at $10 intervals and therefore the max loss potential is greater. On the other hand, SPX Strikes are available at $5 differences which makes controlling max loss to a much smaller level. Which also means that the capital requirement on SPX spread trades is lighter...
As suggested by some of the others here, Trade Small.
Also, staring at the screen throughout the trading day will cause more anxiety.
If you are frustrated by any of the trades you are in, either coz of a lack of movement or too much movement(depending on strategy), enter another trade to take...