Woah, loads of responses! Sorry I didn't get back sooner, I've had a busy couple of days.
Firstly, thank you everyone for your replies, they've been a huge help towards making me feel better!
Secondly, I've come to the conclusion that it is just NRE that's making me have these worries, and the fact that I'd spent 4 days without leaving Sec's side, and hadn't seen Prim since I left.
GalaGirl, you'll be please to know that our engagement is indeed a long one! We don't have a specific engagement end date; we're both in our fourth year of uni (probably doing 5 years each) and then we're moving to Amsterdam for a year, so we probably won't get married till AFTER that (unless we just can't wait any longer and decide to do it in Holland or something)
I suppose one of the things that was getting me so upset (and still is) is the fact that I know there is an ticking clock for me and Sec. Follow up question: how do you guys deal with a relationship that you know is going to end at some point, if you've ever been in that situation?
It's pretty difficult for me and Sec at times, since we are so in love with each other. The problem is that he's mono, so now that he's fallen so hard for me, it's difficult for him knowing there's no real future for us in that way. It's hard for me too, because I'd love more than anything to spend the rest of my life with him in a monoship (but at the same time spend the rest of my life with Prim in a monoship - see the problem?)
I should also point out that though I have a Prim and a Sec, I've grown to consider Sec as more of a second primary... Oh, and in case you're wondering why there's a time limit on our relationship, it's because when we (me and Prim) first started talking about the possibility of a polyship, I made a promise that he would always be my primary, and that after a couple of years we'd become a proper monoship.
And Gala, thank you for your kind words about my maturity, and your congratulations on my engagement
JaneQ, thank you as well for your engagement story, I really enjoyed reading both of them!
[Prim just got home! I'll reply more later!]