It's also reasonable to ask the both of them to be encouraging. Guys are pretty dumb at encouragement off the cuff, but if we're told it'll help, well, we love trying to help.
I mention this because one detail in your story mentioned you walking behind the two of them. Something as simple as asking the bf to walk between you two might help you feel properly included (as opposed to tolerated which you seem to imply). If they don't work to include you, then that tells you something, but don't let your fear shut yourself out.
One of the guidelines my mono, K, and I use is, "no being angry until after you've voiced the problem." This is as much a reminder to communicate as anything else. It also helps us to remember to not let our emotions run away until after we've shared them. Most of the time when one of us is bothered and would be upset, by the time we've finished explaining the problem, the other is already working on a solution. Thus, we rarely get angry with each other. We might be upset by a situation, but if I'm complaining, for example, K lets me know she didn't realize it was an issue, explains how she sees it differently, or offers advice on fixing it. What doesn't happen is this stewing in agony that the OP described. We can be plenty angry after talking about it, but by then things are usually improving.