Hi. Everytime me and my partner go to have sex, when he goes to insert his penis into me I feel a really bad stretching pain?

Foreplay. Assuming he is not "huge" in girth, but even if he is, proper preparation with your mutual choice of techniques / tools (fingers/toys/etc.), can help loosen, lubricate and enlarge the vaginal introitus to be more ready to accept your partner. Lubrication is key, as is condom use to prevent unintended pregnancy and std.