This is kind of an embarassing problem...

Whenever I practice, I sweat profusely from my scalp and forehead. I mean buckets. And it's really distracting for me.

The strange thing is, I'm the only one sweating. When I'm in a classroom setting, and everybody's doing the same poses, how can I be the only one sweating? It makes me feel like I'm doing it wrong.

I know you're just supposed to focus on your breath and let go of stuff like this, but... after a year and a half I'm still sweating as much as I ever did, and I don't know if that's supposed to happen. Could I be approaching my practice the wrong way? Why is my mat underwater when everybody else's is bone-dry?

It could be you are working too hard. Or it could be that you just get hot easily and sweat easily. Are there other group physical activities that you notice you are the 1st to break a sweat? You might also want to check in with a MD to make sure there isn't any medical reasons for this. If you are just warm natured then you can try these cooling pranayamas: http://www.yogabasics.com/practice/sit-cari-pranayama.html
http://www.yogabasics.com/practice/shitali-pranayama.html

Don't worry! I sweat like crazy in class. I'm a beginner, and overweight, so it's natural. I sweat very easily in many conditions and I have learned to accept it. Look at it this way, all those toxins in your body are being released in your sweat.