Tyrotoxicity has some very wise words for you there, reread their posts. Understanding why you're now feeling differently is key to being able to deal with this maturely.
If its about you feeling on some level unimportant, try to find examples of when he's shown you that you ARE important and when he's prioritised your relationship.
FWIW I sometimes do the same. My DP has offered to take me and my DCs on holiday with him and his, but I feel guilty as I can't afford to pay my way, and say no, then end up feeling resentful about it and trying to plaster on a smile when they all talk about it.
Rather than trying to hide how you feel, it might be better to be upfront about it. Say to him that you're glad for him that he's going, but that actually now its happening you are feeling a bit jealous and are wishing you could afford to go on holiday too. Honesty is very important, hiding how you feel never ends well.
It might take some of the power out of those feelings if you name them instead of trying to stuff them down.
This week I've had a similar thing - I texted my DP to tell him that something I'd previously said I was ok about, was actually making me feel a bit resentful and that I just wanted to get it out there rather than stewing on it. He thanked me for being honest and said he'd be mindful of it. However, this has come after many years of us falling out over such things, its taken us a lot of practice to get to this point!
Start now as you mean to go on, being honest and open, taking full responsibility for your own feelings about holidays, money, your relationship etc and deflecting any defensiveness on his part by pointing out that he's done nothing wrong by booking the holiday, but you are also feeling (perfectly naturally) a bit envious. It might make him think twice about being too gushing about it, but make sure he doesn't feel like he can't mention it etc.