You could feel like you're out of love because you haven't physically been around them so in that case the trip might help.

You could feel out of love because you're not getting the attention you expected. Either you have to change your expectations or he's going to have to try harder.

There's probably other things, I don't know your relationship for sure. But never use sunk cost fallacy to engage in anything. If you broke up with him and you have this trip, you might as well not do the trip if you don't feel the love. But, like I said, it could be because you haven't physically been together. Long distance is tricky like that.

Think about it long term: let's say you do take the trip, you realize that being around them is great and everything seems like you should be together. But then you get back together and after the buzz of the trip wears off, you're back to where you started: he's going to continue to be distant (because long distance takes more work than he's willing to put in) and you're going to continue to be unhappy. Unless you can figure out a way to negotiate this and really work on it, your relationship is going to continue to fail.

The fact that he says he still loves you and wants to make it work despite you not feeling like he gives you enough attention means you need to invest your energy elsewhere. You have to figure out how to adjust to his level of investment and he has to figure out how to adjust to yours. If you say he's immature, follow your gut. Immaturity is the reason my relationship ended as well.

I was in a similar situation: My boyfriend and I had two trips planned. One of the tickets I purchased in April to visit him in June and another one was two front row VIP tickets to our favorite artist in August. He just dumped me three days ago and now I'm down at least $1700. Whether or not we're together, I lost that money. Does it matter? Not really. Don't let the money cloud your judgment.