I will disagree with the others a little here. Assuming your 120ED is f/7.5, i.e., the Synta Skywatcher 120ED or equivalent, your focal length is 900mm, which is enough to get decent views of Saturn on a good night. With the 8mm eyepiece, your magnification is 112.5, which is a little low for Saturn, because as you noticed, Saturn is quite small at that magnification. Nevertheless, assuming your optics are in good condition, you should be able to see some detail with that magnification on a good night. On a poor night you won't see any detail at all, and on an average night you won't see a great deal of detail.
The optical resolution of a telescope depends on its aperture (greater aperture means greater resolution), and based on the resolution there is an upper limit to usable magnification. Telescope manufacturers usually state the maximum usable magnification as about twice the aperture in millimeters, or 50 times the aperture in inches. Truthfully, though, this much magnification is very rarely if ever possible. In my experience, magnification of 40 times the aperture in inches (about 190X for your ED120) is more realistic, and even this magnification requires exceptional viewing conditions. You would achieve close to 190X with a 5mm eyepiece, or a 10mm eyepiece with a 2X barlow.
I suspect your experience so far has been hampered by poor atmospheric conditions, compounded by your lack of experience. Keep it up, look for local astronomical seeing forecasts that cover your area, and even on a night when you aren't seeing much detail, keep watching the planet for a while. On an average night, you will find that there may be moments of good seeing mixed in with generally average or poor seeing. Part of learning to view the planets through a telescope is developing the patience to wait for those moments and catch as much as you can when they occur.
On a night several years ago I pointed my telescope at Saturn and was amazed by what I saw - crystal clear, crisp, steady detail like I didn't think was possible with an 8" reflector. It was one of those nights I had heard about, and I spent several hours just viewing Saturn. The next day I visited a local astronomy store and everyone was buzzing - "Did you see Saturn last night? WOW!".
If you want to invest in optics for planetary viewing, I would recommend a good eyepiece more than a barlow. A 5mm orthoscopic eyepiece can be had without breaking the bank, or if you can swing it, a Televue Radian or Nagler is hard to beat. If you decide to get a barlow, keep in mind that a 2X barlow with your 8mm eyepiece will rarely if ever give good views - its just too much magnification for a 120mm telescope. Also, if you decide to get a barlow, you should look for an "ED" barlow to match your telescope's optics.