Please ask me what a minor is doing at a party where everyone is sleeping over?
First off you should all break up. She is too young for an adult relationship and you, from the sounds of it are too immature for an adult relationship.
All of this shows the level of immaturity on both of you and you two should focus on growing up before you commit in an adult relationship.
"She might test me of how far she can go before making me jealous. And are Tobbe still having sex with Kriss? Why are Kriss acting like that now when she was not like this before? "
- She is 17 that is why is acting like this. She is not ready for an adult commitment - you do not test someone if you are in a true adult relationship.
"One thing is for sure... I will not say sorry because then I come across as too insecure and that turns her off. Do you have any advice on what I could do?"
- You will not say sorry - you are afraid of coming across as insecure - this shows your level of maturity. Someone that is mature will say sorry if they should and will not worry about how they come across other than being truthful.
Neither one of you are ready for this level of commitment.