First things first, lol auto's.
Just kiddin', had to do it.
What Mustang did you ride in that now yours feels like a slouch? If it was another auto trans 96-98 GT then it's a pretty fair comparison. If it was a 5 speed 99-04 car then yes, your car is a slouch by comparison.
All comparing mangos to guavas or whatever aside, the best things you can do for your car are as follows : 1) PI swap. If you can do the work yourself, you should be able to accomplish this for under $1000. 2) Gears. Don't listen to anybody's ******** about 3.73's and gas mileage, blah blah blah. 4.10's or 4.30's is where it's at. I know what I said sounds narrow minded, but it's the truth. Parts + install should run somewhere around $700. 3) Off road h or x exhaust midpipe. This is a cheap one and easy to d.i.y.; a decent one can be had for around $150. There's obviously a lot more you can do, but those would be the ones on my "hot" list to get out of the way before anything else.
Do your homework, as I only gave you a simplified version of those 3 things, but if you do those and start thinking about mild suspension upgrades soon afterward, you will run circles around mildly modded 99-04 Mustangs. Come up with a game plan or goal for your car, figure out what your budget is, and when you have those two things locked down reveal them to us here and we can begin to get more specific about what parts will work best for you.