Please help: I can’t ejaculate inside my partner’s vagina during vaginal sex. I am only able to ejaculate when masturbating

Please help: I can’t ejaculate inside my partner’s vagina during vaginal sex. I am only able to ejaculate when masturbating I want to be able to ejaculate inside my partner’s vagina. Please help me with what I need to do. Thank you in advance for your help.