You're not "stuck" at anything with only two months experience. You are perhaps "currently" doing 12 mph, but that will change.

Now. Back to basics. The purpose of gearing on a bike is the same as gearing on a motor vehicle ... every engine has its ideal engine rpm, and the gears are used to keep it turning over at that ideal rpm regardless of circumstance. It has been determined from over 100 years of cycling experiments and experience that the human engine's ideal rpm is 80 to 100 pedal revolutions per minute. That means up the mountain at 80 - 100 rpm's, and down the mountain at 80 - 100 rpm's. Obviously, your speed will vary greatly, but you will be running at maximum efficiency on both sides of the climb.

In order to achieve that, you have to do two things. First, grasp the concept of spinning. Try this: tighten up your biceps, forearms, and fists and make a motion like you were pounding on a table at about 50 rpms. No, I mean really tighten them up, even get your shoulders into it, just like you do when you're pedaling in a high gear at low rpm's. Work it! And your muscles will be aching in 3 minutes or so. Plus, bicycles aren't pedaled with your shoulders! But just watch anybody pushing a big gear at low r's on their bike. They're not only wearing themselves out, but they can't stay on the white line to save their lives! Which, in a lot of cases, it may well be that serious. Like, you know, in Atlanta traffic. Alternatively, when you spin a low gear, your upper body remains perfectly still. Only your legs are moving. No wasted movement, no wasted energy, and the ability to steer the bike perfectly straight.

Next, just take your forearms, relaxed, and move them loosely in that same pattern at about 100 rpms. You can do that all day. And that concept is what they mean by "spinning" in cycling. You spin a low gear instead of pushing a high gear. But in order to do that, you have to grasp the basic "duh!" concept of bicycle engineering, i.e., pedals move in a circle, not up and down. And in order to spin at 100 rpms, your feet have to follow the pedals throughout their complete revolution. That is to say, push down, pull back, lift up, and push forward with equal pressure. Aahh! Now you're using 4 sets of muscles in your legs instead of just one. Much more efficient! But, (there's always a "but", isn't there?), you can't do that (completely) without the second thing I mentioned.

You have to get clipped! OK, you don't have to get clipped. You can get strapped instead! Either way, you have to do something to hold your feet onto the pedals or you can't fully follow the pedals throughout their full revolution with equal force. The old school way was toe clips and straps that you had to reach down and tighten or loosen at every stop & start , and the current way is what they are calling "clipless pedals" where all you have to do is twist your foot a little sideways to release. Either one works, but the differences and what to watch out for is a whole `nother discussion.

But once you grasp the concept of spinning and your foot becomes part of the pedal, you'll start to see yourself improve. Hope some of this helps.

P.S. Cyclometers with cadence sensors are great. But strapping a watch with a second hand on the handlebars and counting your right leg every time in hits bottom does the same thing.