A lot, but I'll say what my most learned and built over my entire trading life.
First, I practiced not to trade all the time, you need a perfect time to trade, not all the time there are good entries, waiting for the right time and opportunity is good.
Second is also connected with the first, which is always not chasing your losses if you already got stopped-out or already closed your trade with profits. Take time to savor your profits and wait again for another entry, there is still a lot of better trade opportunity to come. For me, these are the most important.