Those level 40 players have hundreds of hours more experience playing than you do. Also when they first started playing the stages available were much simpler.
As for equipment. In terms of buff to ability on gear a main is worth 10 arbitrary points while a sub is worth around 3. That means they have around 56 arbitrary points compared to your 39. It isn't such a big gap considering that.
Keep inking and buying new gear. If you get splatted a lot quick respawn will help you greatly. Early on money is pretty tight with lots of new gear to buy so you have to decide between wanting to try out a different weapon or buying different gear to level up.
Also what kind of scores are you getting at the end of a match?
As a roller user using ink flicks for a short ranged is as useful as rolling at times (flicks require a lot more accuracy then they used to though). Rolling is best for ambushes and ink coverage.
If your aim is good you might want to try the Gal series of guns (although I don't think they unlock until higher level) and squelchers. These have good range and are pretty powerful (Gal is a 2 hit KO, 52 fires faster, has shorter range and defence turn it into a 3 hit KO). If you can lay down enough ink the bubbler of the splattershot junior is great for combat (though you have to know when to retreat as knockback is a thing).
One final tip is be cautious about using super jump. It so frustrating when you're about to be splatted and your freshly respwaned team mates think it is great to jump to you and you seem them get splatted on the kill cam.
I'm pretty shitty with the GAL, despite having good accuracy, I get ganked very easily with higher fire rate weapons.