First of all, there is probably a good reason that your percussion instructor (if you have one) wants you to all play matched grip. I've taught at least a couple of lines that wanted to play trad, but I forbid it because they were all really bad at it. They just wanted to do it because it looks cool, but it's easier for me to clean matched grip than it is to fix clean trad.
As for your particular problems...
First of all, you know what your problems and bad habits are, so you are already waaaaay
ahead of the game. I can't tell you how difficult it is to work with a student who doesn't know what they're doing wrong in the first place.
First, I'll go over how to deal with your left hand tensing up. The problem is that not having played matched grip for so long, you have lost strength and endurance and they only way to fix that is to start back at the beginning. And that means legato strokes, or eights. Set a metronome to a slow tempo and play eight on a hand at twelve inches. Ideally, you should do this in front of a mirror so you can observe your technique and fix any bad habits. As you get more comfortable, gradually increase the tempo on the metronome and decrease your stick height. If you start to tense up, back down on the tempo. After you get comfortable with this, try the same routine with triplets (12 on a hand).
While you are doing this, pay VERY close attention to your basic technique. Make sure that all your fingers are on the stick and you aren't using your arms. Everything stroke should be coming from the fingers and wrist.
As for the double strokes, I would suggest playing a double beat exercise. The one that I typically use is: (bear with me here...)
one-e-a two-and-a e-and four-and
Play it once with the right hand, then one with the left, then once with both. After that, play a measure of straight sixteenth note double strokes. Then....
one-e-and two-e-and three-e-and four-e-and
Again, one with the right hand and once with the left and then with both. Followed by a measure of straight sixteenth triples with two doubles at the end: rrrlllrrrlllrrll r
Altogether, it should go something like this:
rr rr rr rr r r ll ll ll ll l l bb bb bb bb b b rrllrrllrrllrrll
rrr rrr rrr r r lll lll lll l l bbb bbb bbb b b rrrlllrrrlllrrll r
I hope that makes sense.
Like the other exercise, you should start high and slow and gradually get lower and faster. I would stay away from playing roll exercises as much as possible until you can play the doubles exercise comfortably, with excellent (preferably 'perfect') technique.
If you work on these as much as possible (if you're serious, I would say on hour a day at the VERY least) it should fix most, if not all, of your problems in no time.
Good luck!