It's ok... as long as your trading amount is just a portion of your entire bankroll! So if you made a mistake and lost a big part of your trading balance, try to learn from that mistake, add more money, and continue with trading! If you learned something you will not fall on the same mistake again! It's how we build experience and how we get better with time...
If someone constantly loses in trading then the answer should be simple, find some other business opportunity!