this is where you learn how to indulge. indulging doesn't mean you get beer and chips and tortillas and extra cheese. well, you can have all of those things at once, but it better be a once or twice a year kind of splurge.
but back to indulging. you pick one or two of those things. look for fajitas without the tortillas as a good base meal and pick one thing you want to indulge in, say the beer. or have the quesadillas, but skip the chips and the beer and the extras.
in our society we are so used to instant gratification and being manipulated into thinking this is the last chance we have to get chips/tortilla/beer/cheese/butter/etc that we have to have it all, all the time. but we really don't. pick what you want -this time- and enjoy that. odds are the restaurant will still be there next week and you can have something else to try and enjoy. and even if the restaurant were to close tomorrow is there really not another mexican restaurant nearby that has similar food? does the internet not have recipes for mexican food that you can make at home? do bookstores and the library not have mexican cookbooks you could get for recipes?
and choosing to not have everything today is fine. it doesn't mean you can't have them, it just means that right now you are choosing to skip some of them. it's not like they won't be there the next time.
-google first. ask questions later.