Jen hun, I think your last paragraph really explains how you feel. It IS disappointing for you, but be happy that you have such a great guy who puts his child first and foremost. That shows what a committed parent he is (always a good trait).
Try and accept that you aren't second best - it's just that ALL children change relationships, whether it's your own child or a partner's. If you haven't already booked your holiday, then i'd use the money you've so carefully saved, and take a week somewhere else - even if it's in Britain. Then NEXT year, make arrangements with your partner's ex. so that holidays don'#t coincide.