As always, when you beginn to look at leaks, look at the street that is causing the leak. If you think your problem is postflop, you're in trouble.
Simply put, you are a calling station preflop.
21/14, the gap is wayyyy to high. 21/18-16 would be appropriate.
What does that mean and how does that cause problems?
You play tight, which is ok at 2nl, but instead of raising the few hands you play you call with them.
So without knowing your playstyle, that can mean two things. You either limp a lot, which is a no go, or you are playing too many hands out of position.
Either way, the problem is that your range is capped, meaning your opponent can exclude the strongest hands from your range, cause you would have raised them. Thus you are an easy target for cbets, cause your range is weak and you dont have position.
So before you even think about fixing your postflop game, learning to defend against cbets and other advanced stuff, you seriously need to buff your preflop game since it is the root cause of your problem.
Your pattern - meaning weasling in behind the preflop raiser to see a flop and then playing fit or fold - is highly exploitable by anyone who has half a brain.
The best players have a preflop raising range between 17-20% but a preflop calling range of 7-8%. Think about that.
The only position you should think about calling is the BB, cause you get good
pot odds. Any other position you should either raise, 3bet or fold.
You are not agressive, you are most likely overplaying top pair hands which is why you went to showdown too much and lost more than half of them. This is a classic low stakes mistake and can be fixed easily by checking middle hands like top pair weak kicker, middle/bottom pair and betting TPTK for value and air for bluffing.
Preflop will be more work, but you absolutely work on that gap between vpip/pfr. As rake still plays a major role in the micros, look to get close to 19/17 or 21/18 max. As you climb the stakes, you can open your calling range on the button and the gap will be wider.