I hate to be the bearer of bad news, but you cannot get a conviction off of your record. The only way to get the conviction off your record is to be pardoned by the govenor. Good luck trying to get a pardon for Rick Perry!
Go the the website, Texas Board of Pardons and Paroles. It explains the process of requesting a pardon. I've had some clients get pardons and then an expunction. One even had an assault pardoned.
At this time and in the position of your case, you cannot get an expunction nor nondisclosure. It is VERY difficult to get a pardon, particularly under the current regime. However, you may be able to get a pardone and it because of your age at the time you got into trouble, it sounds like you are likely as anyone to get one.