The solution, from my perspective, is short and clear. Moreover it is based on the things you can control, rather than growing increasingly dependent on the things which you cannot control.
STEP 1: Sit Tommer (Tom + Summer together as "Tommer") and explain how happy you are for their strong relational growth together. Also explain that, in the recent past, making plans and keeping to them has become difficult and emotionally uncomfortable for you.
STEP 2: Explain that what is most important for you is that everyone's relationship together to not get further strained. Be sure to emphasize that this is genuinely the objective in why you are spending time talking about this issue.
STEP 3: Lay out how you will be changing your trip planning in order to accomplish that goal. -- This is important because ultimately you will need to make changes you feel comfortable with going forward. Decide in advance how 'loosely coupled' your plans can be with theirs, how much schedule flexibility you are willing to work within, how plan changes would be dealt with ideally, and what less-than-ideal circumstances you personally are unwilling to continue participating in.
STEP 4: Ask them to consider how they would like to plan for shared travel in the future. Be sure to listen to what their wants are and consider how these match up with yours.
LAST STEP: Practice what you have talked about on small, low-risk events first -- like pizza and a movie out with friends -- and work up to that month-long trip out-of-country as a gang.
Putting it all together doesn't have to be long-winded or oratory, here's a brief example to drive this home:
They guys, thanks for coming and grabbing coffee with me, because I wanted to run something by you. I'm smiling from ear-to-ear when I see how happy you two make each other, and it's great being friends with you both. When we make travel plans, I get frustrated and upset when those plans suddenly change...which is really difficult on our friendship from my perspective.
I want to make sure our friendship stays strong, so don't be concerned if I book my own hotel, rent my own car, or make my own reservations next time. That way, even if your plans suddenly change, I can keep my travel schedule unchanged -- it'll will be way less stressful on our friendship because I just can't keep up with your last minute changes.
I really do want to consider what is in everyone's best interest, so what else can we all do to make shared travel planning the most fun and least stressful?
Ok, those sound like good things to include next time we plan something. And speaking of next time, there is this food truck event at that roadside bar happening in two weeks, have you already made plans for the 18th? or can we commit to heading out there on that Saturday?
