break up or stay with him

My boyfriend wants us to get married but I dont want to marry him, I love him but I don't see us being together, I have been with him since y9 and we are now in y11, I want to live a single life and experience flirting and dating, but I know I will break his heart and he's suicidal so I don't want to cause him any pain but he's not the one. I don't know what to do ???