Sex hurts (help)

My girlfriend (16F) experiences pain during the start of intercourse. When first inserting during vaginal sex, I (16M) try to go slow as to not hurt her since she is very tight. We are both each others first times and just started having sex about 3 months ago. Usually it just takes some time for her vagina to open up to my penis. We are very open with each other and have tried different things such as using various condom brands and water serving as lubricant. On one occasion we couldn’t even do the deed at all because of how much pain she was in. Even sticking my finger in felt like she was (being stabbed) I have read that this could be a condition or something of the sort. Any tips or anything at all? Thanks