I found out I have mono this morning, but I'm pretty sure I've already had it for two weeks because I haven't been feeling well. I was researching and I found out that your spleen can swell when you have mono and now I'm really nervous that it's going to rupture so I'm being super careful. how likely is it that my spleen will rupture. Would it rupture during sex?

Keep sex to a minimum, and keep it simple. Listen to your body- if you get too tired or your side starts to hurt (even a little), stop.
You can get back to full activity usually in about 3-6 months.
That being said, go to a doctor. After all, mono is a disease.