Dont compare difference with 900 to 1200 vs 1200 to 1400, latter is ten times bigger difference. You can understand this by comparing an average gm with any (ex) world champion.
At beginner level (<1500) can most of players win 200-300 ratings easily in a years work, but Rowson has said in his book that in his 10 years previous book he set up his goal to win 100 point in 10 years but he failed it.
Since chess.com blitz ratings doesnt show your real rating. And most beginners just spend all of their time solving tactics and playing, the games were decided on the fact that who spotted the tactic and who missed. Well i looked at your game i admit he played better than you that game, but not because he is underrated player. He tried to fight for iniatiative (very amateurly) but you didnt castle and went after material. I believe anyone can think about winning material and if you thought that would make you 300 points better player than your opponent you were wrong, you have prove you are better every game you play, not that he has to prove he is worse. You need to be playing prophilacticly and sound. Did you obey principles ? No. Did you ever think about what your opponent wants to do ? No. Then why you believe you deserved winning ?
I don't believe I deserved to win that game. But I also felt my opponent was uncannily good.... for his rating.
On something of a tangent, I was struck by the comments about learning from our losses. (Kleelof) On surface it seems so relevant and obvious, but I wonder how a couple of you analyse your own game losses...eg. do you set up a 3d board and set or use the software on your laptops... take notes, keep a binder of games, sort mistakes by types...? Probably as many approaches as there are players. Anybody want to mention how they face self analysis?
Yeah, serious games I'll go over on a board. I need to keep a better list of mistakes though (some people are better about that).
Basically I try to find when the eval changed. E.g. start at a position you know you're worse, then work backward one move at a time to find the moment it changed. This can be tricky. Moves you thought were good can end up bad, and moves you thought were bad can end up good so it's best to keep an open mind.
After I explore it I put it in an engine. Some people say to wait longer (multiple analyses). At least one guy told me to never use an engine ever.
When you have a list, and you notice common mistakes, it's good to think about why you're making them, and how you can fix it.
Recently I noticed when I have a small advantage I tend to favor a queen trade in positions where I should keep queens on the board. I think I do it because I tend to think "when ahead, trade down into an endgame." To fix it I remind myself to critically compare my middlegame trumps to my would-be endgame trumps and clearly define the role my queen plays before I trade.