One thing I've noticed from spectating some high elo (high diamond / master tier) games, and what I've seen at my elo (gold) is how people react to someone feeding.
Well, first off, in higher elo people don't feed like that. They die once. MAYBE twice. Then they play super passively and only die to organized tower-dives after that point (at least during laning phase).
However, the other major difference is the teammates reaction.
If a lane dies between levels 1-3, the lane is LOST unless there is intervention. If you let that person return to lane for more than 30 seconds without helping them, odds are they will die again.
The high elo junglers realize this and gank that lane immediately. You can't let the enemy push their advantage. Low elo junglers often disregard the lane and try to gank for a "better player" because they want a higher chance at a kill to get themselves rolling. That's fine, but then you're in a race to see who gets more fed and can carry. if you don't like carrying 1v1 against another fed player, don't pick that option. Camp the lane that died immediately and get them back in to it. If you don't, the enemy gets a 2nd or 3rd kill and suddenly now when you want to gank you both die 2v1.
Early kills snowball hard. The 400g is bad enough, but it's really the half-level of xp and your ally's lost xp. If I die 1 minion from hitting level 2, my enemy just jumped to level 2 with half xp towards three, pushed the wave in to hit level 3 and make me miss more. Now I'm returning to lane at level 1 against a level 3. He recalls to buy and I get up to level 2, but he's still got a solid lead on me. That advantage will snowball without immediate intervention.
Basically, the good players recognize that a lane will snowball and stop it before it's too late. You need to do the same. Don't wait for someone to be down 0/3 before you try to help them. You need to do it earlier (or your jungler does) when they're 0/1. Even if you're successful in ganking the 0/3/0 lane, putting him to 0/3/1 and his opponent to 3/1/0 isn't going to change the dynamic of the lane. Even if you do it twice more and it's 1/3/2 to 3/3/0, that lane is still going to be really lopsided (mostly because you've shared the kill xp each time while your enemy got it to themselves).