when I start to play trumpet, and I learned it by myself, I made the same mistake. I positioned my mouthpiece solo that I said in the read of the upper lip.
As you know, and as you experienced, that is the wrong way, because there’s always a little bit pressure on the lip so I can give you all the follow advice. Be happy that you are able to play up to G above the stuff.
And repeat this singing C up to that G not higher.
Try to find the sensation and the feeling of the corner movement when you play higher. I’m sure that your range will develop when you are ready for that you have to be patient.
Maybe you do now the opposite end place your mouthpiece too high. I don’t know you can experiment with it or with a visualiser, but when you are able to play up to g above the stuff then there are things which are right now.
If you have any further questions please contact me via email.