Ok, you asked for more detail

Here is the basic problem:
Once you start to squeeze your throat to sing, you enter a vicious cycle. Increasing volume and pitch requires you to squeeze harder. But that requires pushing more air. And pushing more air also requires squeezing harder to hold it back. Very soon you reach your physical limits, and you can't sing any louder or higher. Not only that, but all that squeezing and pushing makes your voice sound bad.
So to deal with that, you need to start by releasing all that tension. At the same time you need to teach the tiny muscles inside your larynx to connect on their own, without squeezing your throat. Those tiny muscles only need a tiny amount of air to work - any more and they'll just blow open. Or more likely, your throat will squeeze to compensate and you're back in the vicious cycle again.
The lip trills work by doing several things
- Blow too much air and your lips won't bubble
- Your lips absorb a bit of pressure. This means you can blow a bit too much air, but still connect your vocal cords without squeezing your throat.
- It doesn't feel like singing, so you can let go of some of your bad habits
- Excess tension in your face and mouth will also interfere with bubbling, so it forces you to relax there too.
Anyway, did Daniel get you to do the lip trills? If they don't work, there's still other things that might. Lip trills alone aren't enough for me to release (my bad habits are BAD)