20: Too early for G2, I think. You should be happy to let him crawl along the second line at this stage of the game, so unless you are certain that you can kill him if he doesn't respond (if you can't, it's not sente), don't play it yet. Harassing d10 is a higher priority.
28: Same comment as move 20. Note that your variation is wrong... even after W captures, if B does nothing it's seki.
44: Read out the knight's move (o9). Black's stones have no base: capturing them firmly would be fine, if you can, but attacking them at a distance is a more sound strategy in general.
The sequence through 100 strikes me as pretty bad for you (relative to what you could have hoped for). Weak black stones at k4 and d10 are connected and 120% safe.
From this game I might guess that the main difference between crushing DDK's and getting crushed by SDK's is that you make lots of medium-sized gote moves, which the former treat as sente, and which the latter ignore. When your opponent responds to a gote move, that is a huge gain for you; but if you play gote moves too early, your opponent will always have the initiative.
But it also seems that you haven't learned yet how to subtly apply pressure to weak enemy groups, and make huge profits while your opponent saves them. "Attack and Defense" from Kiseido is really good for that.
It's hard to say anything definitive, though.
was a 3-3 invasion. Stronger players won't give you gifts like that. Why don't you post a couple of the games that you lost?
makes a better ko for Black, as a rule. 
, black would not play 
but would tenuki, leaving a seki shape. For some reason I also thought white had a kill at first, but (at least by 
if Black wins the ko.