Bad posture

I have really bad posture and I want to change it but I don’t know how. It’s not just a case of trying to stand up straight because it hurts to stand up straight. My posture causes me to have a lot of back pain from time to time. I’m embarrassed of it, i look at myself in the mirror and I look awful. My friend even said I look like quasimodo (hunchback of notre dame) gone wrong😢. I am trying to do exercises that I found on youtube for it but I don’t know if it’s working. What can I do?