I see a lot of newer players struggling to hit level 15, and many others who hit level 15 and then stop progressing, so I'm going to offer up some quick and dirty bullet points on how to get over that hump, corrective strategies for your approach, and general tips. If any other experienced multi-wipe players (or even first wipe players who managed to hit level 42) have some tips, feel free to add them.

HOW TO GET UNSTUCK

LEARN THE MAPS. A single offline raid with no enemies will do more to enrich your understanding of the map than a week's worth of short-lived PVP raids. I encourage you to walk through every map once and actually try to extract in an offline raid. It'll take a few hours, but you'll understand the maps so much better that way.
Do your quests. DO YOUR QUESTS. DO YOUR GODDAMNED QUESTS. There is no quicker way to level up your character than to just do your quests.
DO YOUR QUESTS QUICKLY. I know there's a 45-minute timer up there in the corner. Ignore it. Your single-stage quests should be done in the first ten minutes and you should be out and back to the menu.
READ HOW TO DO YOUR QUEST BEFORE YOU LOAD IN. You should have a pretty clear understanding of exactly where you need to go. Read the wiki. It's not cheating, it's literally how you have to play this game in a beta state. A lot of the quest descriptor text is totally indecipherable and would take you months to figure out on your own.
DON'T PUSH UNWINNABLE SITUATIONS. If you have to do a quest that is in a specific area but it's hot or being held by a high level player? Give up on it. Extract, try again. There is always another raid and another opportunity to do your quest. There's a good chance that next time, no one will be there and you'll be able to do the quest easily AND keep all your gear.
WHEN YOU QUEST, ONLY DO YOUR QUEST. Don't go out of your way for loot, don't dawdle or wander around looking for PVP. Get in, do your quest, survive and extract.
PLAY SNEAKY. A lot of you likely got into this game because you saw a big streamer playing it and decided it was fun. You don't get to play like them just yet. What you don't know is that they have many, many thousands of hours in this game, they power-level (usually following the tips I've outlined here) and have access to the best kits and generally have god-tier game knowledge. You are a baby hatchling who will get shot from so many places you didn't even know you could get shot from that it will feel unfair. Take your time, be patient, don't make a lot of noise.
DON'T PUSH PVP IF YOU DON'T HAVE TO. Early Tarkov quests are all fun and games. Place a marker here, discover this there. Mid-to-late game Tarkov quests are ALL pvp, and there will be so much pvp that you'll eventually get sick of it. It's better to spend your early levels just getting your character competitive. There will be lots of time for PVP down the road.
STOP PLAYING SO MUCH FACTORY. I know it's fun. There are very few quests on Factory. Stop playing factory. It feels like 90% of the clips I see on this sub are from factory, and a lot of the dogtags I find on factory are from low level (lower than 30 is low level) players.
STOP DOING SO MANY SCAV RUNS. I know they feel safe, they teach you the map, they make you a lot of money, etc. Scav runs don't count towards your overall progress. Having all the money in the world doesn't mean shit if you can only spend it on bad ammo and trash guns. My first wipe, my scav was almost the same level as my PMC. These days I only use my scav as a cheap-ass scav case. I load into factory and run straight to extract. It's much more effective to loot with your PMC and you gain actual experience for it too.
LEVEL UP YOUR TRADERS. I know you make more money selling stuff on the flea. You should still sell as much stuff as possible to your traders. Your traders will sell you the best stuff for substantially less than what you'd pay for it on the flea, and as of this wipe you can ONLY get a lot of the good stuff in reasonable quantities from the traders.
BUILD YOUR HIDEOUT. You can craft good ammo earlier than you can buy it in your hideout, plus you can straight-up craft a lot of quest items in your hideout a lot easier and a lot faster than you could by finding it. Any money that you make from scav runs and quests should go right back into your hideout and your traders.
BUY THE BIGGEST BACKPACK YOU CAN FOR LOOT RUNS. I know it's expensive. They pay for themselves. Every square is worth at least 10-15k. Good example early on: An MBSS is 12K roubles, while a Berkut is 23k. However, the Berkut has four more squares, meaning that the first extra square pays for the berkut, and the next three are pure profit. A T20 Grappa will make 40k more than a Berkut and 90k more than the MBSS for only 30k more in price. For an additional 10k over the Grappa, you can get an F5 that will make you at least 140k more than an MBSS for only 40k extra. The value inside backpacks increases at a huge rate while the purchase prices increases slowly.
DROP YOUR BACKPACK SOMEWHERE SAFE BEFORE FIGHTS. If you know that you're heading into a hotspot, drop your backpack in a bush or somewhere you can reliably find it. You'll be lighter, faster and harder to spot. Double-tap Z to quick drop your backpack. Plus, if you die, then the bad guy doesn't get a giant backpack to carry out all of your gear plus any loot you might have.
TAKE PAINKILLERS BEFORE FIGHTS. It's worthless taking it after you've already been hit. If you feel like you're heading into an area with possible baddies, take a painkiller. The only real risk to being on painkillers all raid is dehydration and starvation, so bring rations or find them.
DON'T OBSESS OVER GEAR. You will die right after wearing your first slick, and it won't feel good. It actually might feel worse than getting killed by someone wearing a slick, because at least you stood a chance. Gear will absolutely increase your odds of survival, but it won't guarantee it. Balancing economy in mind, it can be better to run a cheap kit with minimal parts but GOOD AMMO that you can afford to lose over an expensive kit and mid-tier ammo. I've done entire nights where I use a level 4 digital flora armor, a ratnik and an op-sks with a PSO Scope and survived multiple times. I've also had plenty of very successful raids where I went in with literally no armor and just a rig and backpack and was just fine. How often do you do a successful scav run without any armor? Frequently? Don't obsess over gear.
DON'T SPRINT SO MUCH. I know this is counter-intuitive to the GET TASKS DONE QUICKLY, but there isn't a single map that you can't walk across in under eight minutes. You really don't have to sprint as much as you think. This isn't pubg or warzone, you aren't going to die to the gas. But what you WILL die from is making a ton of noise, making yourself super-visible (the running animation makes your legs especially visible from a distance), and the animation delay where you have to stop running, raise your gun and then ADS. Walk. Call it chad powerwalking. You will survive way, way more if you walk than if you sprint everywhere. Generally I'll spend the first 90 seconds of a raid sprinting (30 seconds to drain my stam, 30 seconds to fully regain it, 30 seconds to drain it again) just to get out of spawn, but then I'll pretty much walk everywhere. The maps are a lot smaller than you'd think, and sprinting is a lot slower than you'd think too.