When we do certain positions it feels like someone is stabbing me
Hi, and thanks for your question. Pain during sex is, unfortunately, not uncommon but there are plenty of things that you can do about it. It's great that you're looking after your health and ready to deal with this issue. It can be sorted out and you can have a much more comfortable (and enjoyable!) sex life.
First of all, you said that you are certain it's not an STI? Have you been tested? This is the only way you can know whether you have one or not, because most STIs have absolutely no symptoms. Going for an STI test is easy, and can quickly rule out if this is an issue.
If you've already done this and know that you don't have an STI then try the next couple of tricks. If not, get that test done first because this is one of the most common reasons for pain during sex.
The second thing for you to think about is whether or not you're fully aroused when you're having sex. Arousal includes lots of feelings: shorter breath, flushed skin, "wetness" of genitals, sensitive skin, etc. It also makes the vagina relaxed so that entry is easier, and lubricated to make it more comfortable. The upper part of the vagina also expands a bit during arousal, making penetration easier. Things that can get in the way of arousal can include relationship issues, stress, distractions and discomfort. Are any of these things happening for you?
If the above two things aren't making a difference for you, it might just be a matter of physics. By this I mean that sometimes your anatomy isn't made for certain things, including certain positions. Sometimes very deep vaginal penetration can cause the penis to hit the woman's cervix, which is a painful experience. The only way to fix this is to stop penetrating so deeply. So, if certain sexual positions are causing you pain and you've ruled out STIs and arousal, then they might just be painful sexual positions.
I've given you a link below to an article that you might like to read about this. You can find a youth-friendly health clinic near you in the Somazone services directory, to make sure that you are STI-free or to treat any that you might have. They'll also be able to answer any more questions that you might have about painful sex. Take care, and good luck.