I have never given advice, but I do hope you find peace and solace in your situation. I'm sorry you are going through any of this.
I, too, have struggled with understanding the entire polyamory concept. I am the monogamous spouse, and I can't wrap my brain around it. I can't speak on behalf of your S/O, but know that she is not doing any of it to hurt you. Know that your relationship is important and hopefully something she too values. Know that you are more than enough and perfect in your own right. I guess that's where that thing called confidence kicks in. Be confident in who you are and what you bring to the table. There is nothing wrong with you or even her. She is just wired a different way. You could be her idea of perfect, and she would still seek something else. You are more than enough, and she has presumably been happy with you and still is. You are engaged after all, so she saw something in you that she has yet to see in another. It is not that this other person has something you don't. She is doing this because she feels some want to share her love and life with another person/other people. Some poly minded people say the heart doubles to accommodate the love of others as opposed to being divided amongst those who are fortunate enough to be in their hearts. Time is an unfortunate sacrifice. There are only 24 hours in the day.
Your S/O needs to slow down and give you time to adjust. There are things that need to be talked out. Poly/monogamous relationships can work, but it takes work. First, for her to go from lesbian to bisexual or feeling some attraction to a male, is different for you. That alone is an adjustment. Second, she has come out as poly, and you need time to process what's being said. She needs to reassure you and remember that your relationship is important and deserves attention and consideration. I have already kind of noticed a red flag. When introducing a monogamous partner to this lifestyle, you have to slow down and think about how it is making them feel. You actually need support from her, too. Once the overall shock wears off, boundaries need to be established.
Talk to her when she gets back. Feeling lonely, abandoned, and all of that is not cool. That means for the time she is so into her new thing that she didn't take time to slow down and ask you how you felt about any of this. In the meantime, take some time for yourself. Take up a hobby, complete tasks that you have been putting off, or just busy yourself until you can have that much needed conversation. Formulate questions to ask her upon her return. Be prepared to spell out your needs, limits, and boundaries. Think about whether or not you even want to be in this type of relationship. It's a lot to handle, and you have every right to say, "I love you, but I can't do this." I wish you well and peace. There are some really awesome people on this forum, and they have been where you are.