Stop playing when you are up

[Mythra]

This has been happening to me for a long time.
Poker is a long term game, which means that if you are winning player, more you play (assuming u play good) = more money.

However, I find myself "afraid to lose it back" and I stop playing after being up 3 buy-ins or so.

Just now, I'm up 4 buy-ins in 1 hour and I feel like I need to stop right now and finish the day +$$.

How can I change this mindset?