Experience. I equate it to like driving a car, when I first started driving, I'd be somewhat scared changing lanes, or what to do next. I'd keep two hands on the wheel and constantly check my mirrors, do the speed limit and overthink time and time again. Then as I got more comfortable it started to change, now driving is second nature.
It's the same in terms of relationships, after awhile of dating your just yourself all that rules and jargon and following this and that becomes

., just second nature.
To me I just do what I want to do, if I want to see my girl every day and shes ok with it I'm going to. If I don't I want, However I also know that tomorrow I can get up and get a "this isnt working" text and well its done and over.
So just enjoy what you have, when you go on vacation your not thinking about the day the vacation is going to end, you go out and enjoy your vacation, the same with relationships. Just enjoy it, if it ends it wasnt meant to be and there will be another girl if you want it.
I like my gf but I know if it doesnt work out I've been here before, don't need gps to navigate me through the deep forest, yeah it will sting for a little bit but like everything in the past and life, I'll get over it
to sum it up, go all in, be yourself. And if it doesn'tw ork out just remember you'll be fine