@Antmany2k
Firstly. Don't worry if you use more of less gas than someone else. It's something all new divers do, but the more you fret the more you will consume.
Everyone is different. I dive a 15L steel, my wife a 12L steel. At the end of a dive she will have used 30Bar less than me, which means I've used an additional 900 litres of gas. I have a couple of friends, both instructors. He is 6'5 she 5'0. At the end of the dive they he has used 20bar less (they both dive the same size tanks). The point here is to prove that everyone will use different quantities of gas on a dive.
You are right, with experience come reduced gas consumption. This will be down to you buoyancy and trim being better, less weight required and being more relaxed. Personal fitness does play a part but not so much as people like to make out
When you are a new diver you can be easily stressed by things, normally this manifests itself in a change of buoyancy (normally positive) To a certain extent its a Sub-conscious thing. Your body has a "fight or flight in built reaction" in times of stress your diaphragm drops giving you greater lung capacity (thus more buoyancy)
Now this can be because you feel out of control, or perhaps you are task loaded (that's why divers practice skills) you could be in a current or all manner of things. The more you dive the less this happens which allows you to carry less lead
Breathing: The books always say use a normal breath. You need to find the mid point of your lungs so when you breath in and out there is still capacity at either end to fully exhale or fully inhale. A normal breath is around that point. Also think a rhythm, Breath in... Pause... Breath out... Pause achieve a nice slow rhythm (try a slow count of 5 for each as a start point)
Read this Blog for fine Buoyancy control
Carry out a weight check at the end of every dive and if you are heavy take a little off each time. Be aware at your safety stop how much air you have in your BCD (can you still dump) Ideally you should be slightly heavy.
You have to go slowly it takes time to become relaxed underwater, at first you might get to that place then something disturbs you your buoyancy changes (normally positive) likewise when you worry about your gas contents you talk yourself into bad consumption
Set yourself little milestones, perhaps removing 1kg per dive day or surfacing with 10 bar more than you did before. Know what your contents generally are at say 20 mins (it that's your halfway point ) give yourself a mental pat on the back if you don't reach that gas level until say 25 mins. Or whatever.
The fact is no two people are the same, so don't compare yourself with others, compare yourself with yourself. And most of all, enjoy your diving
