Thank you both; they're all reasonable suggestions. I do definitely try to bring all of these issues up, although given its limits I also want to be sure to not use all our time together on dealing with them. We did actually discuss some of it last night and I'm feeling better, which is generally what happens. And then I feel silly for flaking out about it, until the insecurities resurface, at which point I try not to flake out again but don't always succeed. Sometimes it seems like a switch gets tripped, and once I start through the cycle it's hard to disrupt it; it just needs to run its course.
Part of the issue is that my boyfriend has less capacity to plan than I do, and I am very much a planner. I find routine and knowing what to expect reassuring, and he really tries to accommodate me on this, but sometimes it isn't feasible. He tries to not make plans that he might not be able to keep, so I don't get hurt, and sometimes this means that there's a lot of uncertainty, and sometime they just get broken anyway. But I think we're both making an effort to adapt here.
I did actually think of passing on the letter, but I'm not wild about the formality of that within a conversation. Plus writing is often so useful for me in hashing out my thoughts that I don't necessarily need the letter as a tool once I've written it; I can probably say it in far fewer words. And since I came back and looked at the letter and cringed, both at the screaming insecurities and at the stuffiness of it I'm glad I didn't use it. I may yet pass on a few excerpts that might be useful, but the whole thing would be a bit much.