Is one sinful for masturbating before intercourse to treat premature ejaculation, in order to successfully perform intercourse?

Masturbation is unlawful (haram), and it is even more so for a married person. It is not permissible except in dire necessity, such as someone who fears falling into fornication (zina) if they do not masturbate. What you did by masturbating is unlawful (haram), because premature ejaculation has a cure. So, seek Allah's forgiveness and repent to Him with sincere repentance.