Hi,
I am sorry you are going through this.
I hope I am replying before the party both of you were invited to.
I am unsure why you are feeling anxious but there could be a number of reasons: you mentioned yourself, jealousy. Do you know why you feel jealous? Is it because you have such a close bond with him?
Anyone that is very close to someone else is affected by what they do/where they are.
I'm not sure why you are anxious about being there when he is. You can do a few different things:
You can choose to not attend the party (which would mean giving up some potential fun for you).
You can talk with this friend directly and see his views on you being at this party (remember though that if you talk to him, make your choice ultimately out of what you want to do, not necessarily what he wans (keep the communication open)).
Lastly, you can go to the party and see how things turn out. If he is there, than it is your choice (or his) to choose to hang out with other people or to hang out with each other.
I am glad you are being caucious however, keep in mind that what you want to do should be the most important.
Hopefully this is helpful.
Take care.