You are being too clingy and you are over-reacting. From her point of view.
Put yourself in her shoes.
Even better, put yourself in her fairy godmother's shoes. If you were a wise magical being that was watching over her and appeared now and again to advise and guide her, with you having the perspective of thousands of people's experience, what would you say to her?
You'd say to her that she should look to develop herself as much as she can at uni.
You'd advise her that her time to enjoy life is now. And that her time at uni should be the joint best days of her life.
You'd tell her to live in the here and now. And that if she's presented with a man that's with her now, she should go on an adventure with him now. Instead of putting off that kind of adventure until Christmas, or until she gets a visit from her 6th form boyfriend.
You'd advise her that having the skills and ability to attract men is more important than whether she has a boyfriend at any particular time or not.
You'd advise her to make herself a free agent. And to inform her boyfriend in a diplomatic way that he is now a free agent.
When you really love someone, you want what's best for them. Even if this comes at the expense of short term comfort for you.
When you have a great relationship with someone, you set up win wins. And you avoid win losses. Treating her as a puppet whose strings you are pulling remotely is a win loss. A win for you. A loss for her.
There's also the psychology. The more freedom you give her to explore other men at uni, the more likely she is to come back to you. And the more likely she is to be honest to you about exactly what she's been up to.
Do not argue with her about kissing other men. Do not argue with her about going further than that with other men. Arguing about it makes you seem emotionally weak and therefore less attractive to her.
Thanking her sincerely for telling you about what she's been up to is the way to go. With you being honest about telling her how you feel about it, but being honest in a diplomatic way, such as telling her in a detached tone of voice as if you were talking about the feelings of someone else. Followed by you putting a positive spin on it by saying that it's great from her point of view that she's living uni life to the full, and developing as a person.
So that you have a productive, honest, loving, supportive conversation with her, instead of having an argument.