When I played it some three years back, one needed to find usually some oxygen plants and caves for oxygen refills. I bet core play still remains the same. It really isn’t that hard tbh.
running wears out your life support faster, same with jetpacking, that may be why you're running out. sometimes the rng is just not in your favour, though, so you may need to make a new save just to wind up on a different planet, but i've never had to do this more than once, imo it was worse closer to launch when you were dependent on plutonium for launch fuel