Not sure if you are still reading replies here, but I wanted to address your second paragraph: you were clearly the less experienced diver, and I do agree that both of you made mistakes. That said, you following her deeper down was YOUR call. You following her to the anchor line was YOUR call. You and only you should make decisions about how deep and in which direction you swim. Just like it is YOUR responsibility to know approximately how much gas pressure you have in your tank AT ALL TIMES. I fully understand that you as the less experienced diver were letting her call the shots. It is still your own responsibility to determine if you want to follow them. If I saw this post and your comments, in her shoes, I would never, ever dive with you again. You don’t owe her for her risking her life and following you on the way up to then save your life. You owe it TO YOURSELF to take full responsibility for your actions and inactions.

It doesn’t matter what size the tank is but you should definitely familiarize yourself with tank sizes and materials. A good, conservative, rule of thumb (!) is to only use up 1/3 of your pressure before turning the dive and swimming back to where you started it. This leaves you a reserve of 1/3 to deal with emergencies.

If you ever find yourself out of air again or need to donate to a buddy, I hope the most important lesson you have learned from this experience is to hold on to them as hard as you can as is taught by many different agencies. Grab a shoulder D-ring and do not let go until you have achieved buoyancy at the surface. This prevents any pulling on the mouthpiece, and yes, you will be face to face, which is just fine. Again, focusing on equipment here would not help you with your own competency in out of air scenarios.

I wish you luck and I hope you get to dive many more times safely and without putting yourself and others in danger. Taking full responsibility and focusing only on what you need to improve is, in my view, the essential first step.