I haven't been thru that airport but I just emailed friends who live in Ft Worth- they both said that should be fine as long as first flight is not late.
They both said that delays there are usually only weather related.
You will leave from Terminal D which is the International terminal. I think your LAS flight will come into Terminal E which is next door and is quickly reached by a Skylink Monorail- they run every two minutes. Hope this helps- I would be feeling a bit anxious too,I prefer to be sitting in an airport waiting than stressing at the last minute.
Also- when you get on your flight in LAS Vegas ask the Gate agent to radio ahead to DFW to make sure they know they have passengers arriving for your connecting flight to Cancun.
