2003 Ford Focus

It could be a number of things. Is the check engine light on?
I would check for codes regardless (you may have a misfire). If you have access to a scanner, I would also check sensors input (PIDs).
If you've never got a tune up, I'd check the plugs and wires to make sure you have a strong spark.
If you can, check the fuel pressure. You said you never replaced the fuel filter; the maintenance schedule calls for a replacement every 30,000 miles on Focus...I would replace it if I were you. It's not a hard job but it's a bit of a pain because you have very little clearance to do it. While you're at it, you may want to check your air filter as well if you never replaced it.
Finally, you may have a vacuum leak.