I'm not a fan of claiming "It's never gonna happen".
Starting tennis at 18 is definitely late in terms of ATP aspirations. Many coaches and experts say it takes 10 years of practice to turn pro. But that's such a generalized timeframe. It depends on many, many things.
By not starting so early, your body (presumably) has not be pounded by years of hardcourt training, so you should be a fairly 'young' 18.
Secondly, With passion should come great work ethics, like eating right and realizing you need to spend a lot of time working on your fitness (not bulking up, but your cardio, leg/trunk/core strength). Even top juniors can slack about their eating/sleeping/fitness habits. By being supremely fit, you can match up evenly (physically) no matter how good opponents are.
Look at Pat Etcheberry's fitness program, for example.
And, if you practice the right way, that 10 years can get cut down dramatically.
Learn to serve and PLACE your serve. I have been aced by old men who did NOT have huge serves, but could place their serves beautifully.
The list goes on and on. Just play as often (this must include matchplay! Playing competitively is so different than practice matches) as you can, and try to find a way to afford a coach who shares your passion for the game.
The internet can be a great resource as well. Lastly, watch yourself on videotape -- it helps.
Best of luck...