Ask yourself 3 Questions
1) Is the game good?
2) Is my seat good?
3) Am I playing good?
If the answer to at least 2 of the 3 isn't yes, it's probably time to sit out.
Stop losses are generally to keep people from slipping into poor play, D-Game, because they are tilted, tired or chasing. It's a way to put a number on things like "everytime I lose x buyins I start to play really poorly"
But as you get more experienced, you can tune into when you're playing poorly before you hit your stop loss.
Regards to the whole thing about losing sessions, the key is to really look at every hand of cash game poker as one giant session. And your best results will come in the long run if you're playing when you're at your best.
It's not about having a winning session, or a winning day, a lot of that is just due to social pressures of people asking you 'how you are doing'. Because variance is such a major contributor to the outcome of your short term sessions it really shouldn't matter. How well you are playing, and how good the game is should be the primary determinants of when to quit or keep going. (And Quitting as Tommy Angelo said is one of the most important skills a poker player can have).
It doesn't become easier once you're a reg, it becomes easier as soon as you adopt the long run mindset, and all regs/pros have to adopt it or else they'd go insane
Here's a video on knowing when to quit that may help
https://www.youtube.com/watch?v=k5p72dPik20
And here is a video about the whole graph thing/peak stuck, which I've experienced tons over the years, and in this video I just kind of lay it all on the table
https://www.youtube.com/watch?v=6Z36XsMtSv0