Look at the Liverpool's Man City's when they are attacking. Never mind just them, look at most Prem League teams. You see 6 or 7 players within TV screenshot in and around the opposition penalty box. Sometimes more. That's how they put defences under sustained pressure. They have to get back down the pitch too. Why is it they can do it, but we can't, or won't? We have to learn to defend without needing to keep 8 or 9 players permanently in our own half.
Couple of things...
If you sit really deep, you've got to cover a lot more ground to get forward, and while you're trying your options are limited. Any balls cleared just come back at you.
When you do have the ball you have to pass it forward through midfield and keep possession. No point smashing it forward then expecting 6 or 7 players to chase it down.
For me, if you look at the last few Prem games, look where our actual back 4 sit - sometimes they're on a line across the penalty spot and the midfield are on the edge of the box. It's just too deep and makes keeping possession and getting forward next to impossible.