Is really impossible for anyone to give you any really good advice when we haven't actually heard what you sound like or been able to look at how you are playing. I do know that when I first started teaching my daughters how to play that one had an absolute horrible tone and it took a lot of time to work with her to get it straightened out. I know from trying to teach them both at the same time that what worked for one didn't work for the other... I honestly would say your best bet is to get a good teacher to work with you on your tone specifically. There are a lot of things you can try though. First. A shallow mouthpiece isn't going to help your tone it will make it harder to get a full tone. I have an extremely shallow mouthpiece that is great for the upper register, but it is very difficult playing low. I don't know what you mean when you say you have 1 1/4 as I don't know what brand you are talking about.... Frankly I would probably suggest you use a Schilke 15 and throwaway your shallow mouthpiece. No reason to work on upper register until you get the tone nailed down at the low end. Honestly I think whoever your teacher was to start with did you a great disservice by not getting your town straightened out early on. You may have some bad habits ingrained in your playing at this point... but again someone working with you in person would be better at figuring that out. I would also drop the teachers that suggested how you hold you fingers has anything to do with your tone. For me who cares if you can play a passage fast if you doesn't sound good. Tone should be the only thing you work on until it is to a level that is acceptable to yourself. I also wonder, after you play a for 30 minutes is there a ring visible on your lips from holding the horn? If you are putting too much pressure on your lips with the horn. Something you might try is buzzing your lips without a mouthpiece. All a mouthpiece does is direct the sound from your lips into the trumpet. You should be able to get the same sound that a player gets from just buzzing in a mouthpiece by simply buzzing your lips with none... the upside of working on this is you remove the position of the mouthpiece on your lips as a variable to worry about. You are just using your lips and air nothing else. I am assuming you have already been shown the proper way to breath, using your diaphragm and not your chest... One key to being able to get a good sound is being able to get good air pressure and that means being able top open your throat... Sadly there are just so many things you might be doing wrong that it is nuts to try and figure it out.... but I have never seen anyone that couldn't be taught to get a decent tone.... quite a few couldn't ever get their range where they wanted... but I don't think it is ever impossible to get a good tone.