It's hard to tell from the one picture, but given that your right heel is UP and your right leg's bend is about what I'd expect to see at the bottom of the pedal stroke, I think you might need to lower the seat a little. And yeah, as it seems that it's already pretty low, maybe the bike is a little big for you.
That being said, descending is definitely a skill. You don't just hold on and go wheeeee! (well, maybe a little...)
There are a lot of things you can do to make things more stable/controllable.
First thing, don't tense up. Your hands shouldn't have a death grip on the bars. I like to be in the drops and keep two fingers on each brake, ready. Make sure the other two fingers (on the bar) aren't directly behind the lever.
Next, when hitting a turn at high-ish speed. put the foot on the outside of the turn DOWN. So if you're turning left, put the right foot down and put your weight on it. This helps to keep the tires planted on the ground, carving a line around the curve.
Some will argue for one brake or the other, but I tend to use both brakes during descents. Don't just mash them down and skid down on your brakes -- that'll create heat on your rims and could blow your tires. You have to feel out where your center is (front to back) and feather each brake accordingly to keep it there while keeping your speed where you need/want it. You definitely don't want to be out over your front wheel. I tend to try to get my chin right over my stem by a couple inches (not too close or you'll lose some teeth when you hit that unexpected bump. Since, however, you have a nice loose grip, the bump won't buck you off the bike/out of control, it'll just bounce under you and settle back in, like a mountain bike.)
It helps to be out of the saddle and have your legs acting as shocks, of sorts.
Yes, your neck, hands and legs may begin to ache. Again, descending isn't just a slide. Done right, it's nearly as tough as the climb -- just in a different way.
If you really want to trip out, look of 'death wobble'. Then be sure that while descending on straights, touch the top tube with your knee(s) frequently (or constantly) and also pedal a bit here and there (even though you're spun out beyond the top gear.) These sorts of things can help avoid the bike reaching a resonance and vibrating wildly and killing you.
Oh yeah, and I am no expert, I just live near a lot of hills and find myself descending a lot in some scary spots, so I've read a lot on this forum and watched some quality youtube videos on the subject where actual expert riders throw out gems like these. I highly recommend you seek out some of the same info.
Be safe out there! (that being said, I thoroughly enjoyed blowing away the whole club ride a few weeks ago on a big local descent and hitting 49.5 mph in the process. Talk about adrenaline!)