To rule for yourself and not be subject to authority?

Shalom Rabbi Michi
In one of the lessons – in scholarly thinking – in the first chapters, you claim that it is possible to rule for yourself, in contrast to the “mainstream” ones – you study the issue and this conclusion is the halakha for you.
I’m having fun with it, and it’s also true that the Tosafot authors disagreed about Rashi, but either he “allowed” them or there’s no way there’s such a side.
But on the other hand, first-rate poskim implemented this idea of ​​not rushing to disagree with their rabbis, Rambam versus the Riv, Shulchan Aruch versus the Pillars of Teaching, Rav Elyashiv versus the Chofetz Chaim, Chabad rabbis versus the Shulchan Aruch of the Rav, etc. There are many more.
I ask the substance of the matter, isn’t it in the essence of the Torah that, in questions that have sides here and there, you should make a rule about which rabbi you go with and not rush into his opinion? It is true that there will be many exceptions, but it does sound like there is a line that is drawn that the Torah passes through.
 
thanks
peace