Dear DLJ, You're at the tough age when the pull of family and the pull of your boyfriend/spouse can cause tension in your relationship.
I've Been There...
I've been there, believe me. In fact, it was the main source of tension in my marriage for several years. This is what I learned: Inevitably, if he's the ONE, your loyalty should be with him first.
Are you ready to say he's the ONE...maybe not yet. So, then what?
You're Focusing on What Others Want
Your problem here is that you are focusing on what everyone wants from you and what you think you owe them...producing guilt. You may never be able to put down the burden of the guilt unless you really feel comfortable following your true desires independent of everyone else's expectations. In this case, you may have ambivalence about traveling to either place.
Talk to Your Boss
If it were me, I'd talk to my boss about the dilemma and go to both. Both are essential to you, and the boss might dock your pay for the few days you're gone, but I would feel very obligated to both events.
The other alternative is paying for your ticket to Oregon and not going. Then YOU are in control. I bet you'd feel too guilty doing this, however. I don't think you'll ever stop the guilt. So, how do you decide?
You Need to Decide Which is Most Important
You said that the family reunion was most important, then plan on that one.
You have two choices, pay off/cancel with your boyfriend, or confess to the boss. Which is more anxiety/guilt producing? Do the one that feels best, but feel good that you get to do what you want with your family. You don't have to resent your boyfriend if you make the choice about going yourself. If it's just money, pay him back. If he's hurt, and you don't want him to hurt, tell your boss that this is vital to your relationship.