Help with chess thinking

Here is the game I just played tonight at my local chess club, I was white:



My question is, at move 27, what questions could I have been asking myself that would alert my to the importance of the opening of the c-file? I know my retreat with the knight was weak, you generally should not make retreating moves unless you have good reason. I have a lot of trouble putting away a won game, I always seem to manage to hang myself. Any help would be appreciated!

Btw, I'm happy that I have improved enough to start fathoming middle game plans. My plan here was to swap LSB's (neutralizing the bishop pair) and cementing a knight on the weakened light squares to play a good knight vs. bad bishop position. When I managed that, I felt I was winning. Then, all of a sudden, I was lost. Chess is demeaning in that respect.

the real issue was that your d-pawn became weak. if the knight stayed on f5 you would most likely survive the opening of the c-file without trouble

@Rise Perhaps you're right... I should have gave more weight to the fact that the bishop on a7 pressurizes d4. My retreat was lacing enormously. What a pity. :(

So I could have thought, "what are the possible targets in my position?"

Thing is if you played Kd3 to protect d4 the c file doesn't open at all, as you do Kxd4 at the end. After Kd3 everything is protected and the only possible break to calculate is e4+?, which is a bad move.

"What are your weaknesses?" is always a question to ask yourself. Another question is "How to activate my pieces?" The rook isn't good when protecting a pawn. There's not any open lines yet, but black just played h5, which will open a line. If you recapture and use the h-file or recapture with the f-pawn and use the passed pawn, the rook is more useful over on the h-file. "What pawn levers are possible?" and "which files are going to be open?" are useful in this position.
The real strange part of h5 is that black is missing a piece over there. The bishop isn't going to get to the kingside soon, and, once it does, what is it going to do?
@Rise is correct saying that the c-file shouldn't be a concern as all entry squares are covered, after Kd3.
If black didn't err with h5, one of the plans I would consider, although it looks bad, is f4 to open the e-file, even at the cost of a pawn. Black would be advised to keep the game closed by playing e4, but that knight look good on e3, not enough to win the game. Another plan is to exchange on e5 and use the pressure on d5 to restrict black.

In the position after 26...h5 ask yourself: which white piece is placed best? Which worst? Answers: Nf5 and Ra1. So you should not move your well placed piece, but you should activate your worst piece.