Need advice ASAP

My girl and I have been in a relationship for about 5 years, on and off a little but mostly on. Things were going great as usual in June, she even said she wanted to marry me, I'm the best bf she ever had, she's so happy and only guy her mom ever liked. 2 weeks later we went camping with the whole family. Again, nothing seemed off. She was lovey Dovey, had great sex all 3 nights. Then the day after we got back, she breaks it off out of nowhere. I give her space and we talk a little in the next 2 weeks. We have another talk and it goes much like the first. 2 weeks go by to today and she posts a picture with some guy and hearts in the caption. In less than a month, she's with someone new. Can't believe it. We have a trip booked together for a mural friends destination wedding in November. She wants to change it to separate rooms or discuss the options. I need to know, how do I react to this guy and the picture and what to do for the trip?? I love this girl so much. I know at this point it's going to take time to get her back but that's what I want to do. Any and all advice appreciated! She wants to talk about the trip tonight on the phone