5 inches sounds quite a large bit for a 14.2hh TBx - have you checked the fit of your current bit? Also has she had her teeth & back checked recently? Napping & opening her mouth could be a sign that she's uncomfortable.
The cheapest alternative is to fit rubber bit rings (£2) to your existing bit - this will a) reduce the width a little & b) prevent the bit sliding through her mouth (unless she opens it really wide, in which case she'll have plenty to chew on!). Otherwise, if you're happy with your current bit (& her teeth/back/saddle are ok) I'd just get a cheap noseband - flash or drop - instead of spending out on another bit which may/not work.
Happy riding.