First, congrats to you for your unwavering honestly! I know you feel you are paying a price for this. As a long time manager, may I suggest a few things that just may be happening.
First, an employer has the right in most companies to hire someone with an isolated item or issue who otherwise has excellent references and interviews very well. Everyone makes mistakes, and if there is absolutely nothing else negative on a background check, I would give that person a chance and have done so many times without regret.
Lying about that on any application might get you a job, but when the state or national background check comes in a few weeks after you start working, you would be terminated for failure to disclose the information about the DUI, not necessarily BECAUSE of the DUI.
If, in your application processes, you are simply completing and submitting applications as opposed to personal interview, I would suggest you call the Hiring managers in each place you applied directly and speak to them about your application and strong desire to work in your chosen profession. Try to get a face to face meeting, dress for success, be extremely professional and you will get a shot. Looking someone in the eye and telling them that the DUI was an isolated incident and would never ever occur again would be powerful.
I think your problem is that these prospective employers
only know you on paper and have not met you. Insist on just a few minutes of their time in person, offer to work nights and weekends for awhile if that what is takes to get their attention. Tell them you will earn their trust and be proud to have you on staff. Wishing you luck!