Every time i have sex my vagina splits at the opening .... why is that? and what can i do to prevent it?

May be: Your patner could be to big or rough or your dry. Try lubricant and moisturizer and gentle sex may reduce tear.

Ouch: Typically the elastic in vaginal tissue does accommodate with time and stretch. Sometimes tearing can be due to yeast infection, undetected of course in your case. Other times it can be due to vulvovaginitis of other source. You can try some desitin (zinc oxide diaper cream) or similar rash cream on the area to help heal faster. The vagina itself isn't hopefully tearing, hopefully it's only the vulva. Gentler sex too.