Ok, so the fact that you sang well enough to be a soloist when you were a youngster proves you're not tone deaf or incapable of singing.
The fact that you've then had a sucession of unsatisfactory teachers is very bad luck, rather than inability on your part.
"Support" is a tricky one to explain. And indeed it seems a lot of people try to support the sound in a number of wierd and wonderful ways - which might work for them but don't for anyone else. So here's my own take on "support" - it works for me this way:
For me, support happens when my whole body is physically relaxed and I put all my mental and physical energy into the music.
I don't make conscious muscle movements, and certainly don't do anything to my diaphragm or throat, I put all my concentration into enjoying the sound that comes out - whether or not anyone else does

.
I describe it as singing with the whole of me - not from the neck up, or the diaphragm up, but from the toes up.
If the air coming out from inside me is relaxed and energised, the voice will be supported, otherwise it won't.
If I try to intervene and make my diaphragm or throat or abdominal muscles do something, the result is a nasty, unsupported, strangulated squeak.
My body knows how to breathe rather better than my mind does, so I have to let it get on with it.
One way to feel support is to give a really heavy sigh. Then once your breath has come in again, do the sigh again, but as soon as it's started let a note from the middle of your range come out to "yah".
Imagine that note disappearing off into the ether, and notice how your body feels as the breath begins to run out.
If it's a sensation that every drop of air from your body is going into the sound, then you'll feel and hear real support. And your body will automatically take another breath ready for the next sound.
So the next time, try it without doing the sigh first, just relax the breath out and see what happens......
Pitch is another tricky area. When I sing, I have to trust that the note that comes out will be right - if I do that it usually is.
If I try to intervene and "pitch" the note, I almost always miss.
So it would appear that my body is better at pitch than my mind is!
A similar thing works for me with pronunciation.
If I
try to pronounce vowels when I sing, they'll go wrong.
If I speak the words before I sing them, and then just let them come out the same shape in singing as they did in speaking, with no extra intervention, they will sound better.
Once that's cracked, the fancy technical stuff like diphthongs can be sorted out later, but getting the basics sorted first is important.
This is rather a ramble, but it may give you some ideas to try out. Good luck.
Cheers
Katyjay