May be a difficult problem to solve.
You do not describe the ulcers, but i would assume, as they are the commonest, that you are suffering from Aphthous Ulcers.........not much that one can say other than their cause is ill understood.
Stress, strain, other underlying conditions may contribute to their persistence, but i doubt whether your beta blockers are playing any part at all.
Try 1% triamcinalone in Orabase paste, rubbed on three times a day.
Good luck as treatment is trial and error.