How do you change a core belief?

When I was little, my mom suffered from multiple-sclerosis, and was house-bound and later bedridden, and I believed that my happiness, and enjoyment of things-(physical things, like running, and being with friends), made her jealous, and angry with me, and somehow "hurt her". So I tried to stop showing happiness around her, and eventually it stoped being an act, and became the truth. Well, she's been gone now, for 4 years, and I still believe that it is hurtfull to her, or her memory to enjoy anything, or feel happy. I believe to do so would be like "cheating on her", or not showing true devotion, or love to her. Now that she's gone, I can never ask her what her final wish of me was, be it to suffer, or to be happy-I will never know. For some reason, I feel morally right when I am sufferring, and guilty when I get into joy-producing situations. How do I go about deprogramming a core belief like this? -yes, I know about D.B.T., to me dbt is more of a diagnostic tool, rather than a fix.