There is life after AT&T.
The biggest mistake is to make AT&T your identity. AT&T paid you for to work for them, and maybe helped gain new skills or credentials to do your job, but that was all in the company's interests, not yours. AT&T will not hold your hand on your deathbed, and it will not - and did not - hesitate to dispose of you when for whatever reason they decided they no longer need your services.
If nothing else, prove to yourself, not anyone else, that AT&T management was wrong - success is the best revenge.
OP is @Yc8wFaM-3fwu.