I think he's feeling that he doesn't have his emotions under control--"Ain't no reins on this one"--and that's at least part of what's scaring him.
Yes. Definitely.
You know, that line, "if you can't fix it, you got a stand it," appears twice in the short story: once in the Motel Siesta scene (which is the source of a lot of the dialogue on the camping trip), and once as the final line in the story. A lot of its power as a "life philosophy" comes from being the last line in the story, when it's got all the weight of Jack's death heaped on top of it, and where it sounds to me like a really depressing statement about Ennis's future. But the earlier time, in the Motel, that line comes across in the same way in the movie and the story (except that, in the movie, the rest of the camping scene really does scream "I love you, Jack, even if I can't bring myself to say it" to me).
Did anyone else expect to hear the line without "Jack" in the middle of it? In the trailer, the "Jack" is cut out of that line. (It's just in a short splicing of critical lines from the movie, along with things like "You have
no idea how bad it gets" and "I wish I knew how to quit you.") It jarred me the first time I heard it, and I think I probably didn't think of the significance of the change as a result.