I want to tell my girlfriend she has a bad singing voice
Why do you want to tell your girlfriend she can't sing?
Her singing is terrible and it's painful to listen to. I think she should know.
I know you don't like your girlfriend's singing but telling her how bad she is would hurt her feelings. She may really enjoy singing and you'll just have to tolerate it. If you do say something, make sure you do it in a kind and gentle way.
I said she ugly and a big dick sucker, she cried. Why was that wrong?
That was wrong because you insulted her and made it sound like she was a bad person! You should never say something like that to anyone, especially someone you're in a relationship with. You need to apologize to her, or break up with her and let her find someone who will treat her with the respect she deserves.