Okay, are you working on the visual editor? If so, I'd switch over to the html editor, which I personally find easier to work with.

And then take out the messed up lj-cut code -- I'd suggest ctrl+f 'lj-cut' and taking the stuff out. Then, after your heading, I would put <*lj-cut text="Your text here"*> (taking out the asterisks, of course). And if you want, you can add <*/lj-cut*> at the end of your post, but that's not necessary, LiveJournal puts it in anyway.

I hope that helps! ^^