I've added some comments in the file below. Here are my biggest suggestions:
- Don't start invading until all the big, open sides are claimed. It's fine to let your opponent have a big territory if you can make a bigger one.
- In life-and-death fights, expanding your eyespace should be your #1 priority. For example, at move 80, you tried to seal two black stones in - it would've been much better to expand your eye space with B8.
- Related to the last point: You seem to play solid extensions in a lot of places where a diagonal move could work (see my comments on moves 18 and 110 for examples of what I mean). It requires a little reading, because you need to check that your opponent can't cut it, but it can give you an advantage when it works.
- You're walking away from some fights that should still be fought (see moves 36 and 68). To be honest, this is really a judgement call - sometimes it's right to walk away, if a fight can't be won. The only real suggestion I can give you here is to play lots of games, and eventually you'll learn to judge which fights can still be won, and which will only make your opponent stronger. When in doubt, fight it out - even if you can't read all the variations, that's OK, because your opponent probably can't either. Even if you lose, you'll learn something in the process. If you don't try the fight, you'll never know.
To answer your question about the left group, it's definitely dead at the end of the game. Black can kill it at A12.
The situation at move 25 is a LOT more complicated. I, um, got a little carried away exploring all the variations, way beyond what I would actually be able to read in my head.

The tree gets kind of messy. The good news is, I don't think black can kill you at that point (but anyone can feel free to correct my analysis

)
http://www.lifein19x19.com/forum/download/file.php?id=4686