How do I improve my middle game?

Lately, I have been stagnating. I always analyze my games after playing them and they all seem to be showing an alarming trend. I often times start out well, playing the beginning with an advantage or even. Once the middle game hits though, I begin a steady (sometimes sharp) descent into a disadvantage. I don't even realize it's happening until after the game. Can anyone shed some light onto why this may be happening and/or offer some tips?

Can you post a typical lost game that shows this?

I actually ended up winning this game on time but it pretty clearly shows the mid game descent I was talking about.

judging by this game (you missed 27...Nxg3), i suggest you to do tactics. If you can, buy a big tactics collection book and work through these. Three books are even better ;-)

also, repeat those tactics until you have them all in your brain.

Generally when you feel you have left the opening you should assess the position coldly, examine what is going on, what pieces are doing what, which pieces are not doing anything, do you have potential threats, does your opponent? and so on. Scan the board for weaknesses etc. That's advice someone gave me and it helped.

On move 10 you have completed development. A good plan might be to double the rooks on the d file and use the outposts on d5 and f5 with your knights.

I'm a patzer so that might not be the best plan but that's what I would do.

16. Rb1 stands out to me, the relationship between the black knight, bishop and queen begs for your knight to drop into d5 after taking on f6. Did you consider it?

addon to #4 and #5: this may be of interest to you:

www.google.com/search?q=400+Points+in+400+Days

it is a brutal method and i have not tested it but it may work.

@DeetM I did consider it, what I saw was Nd5 Nxd5 exd5 bxg5, not good.

Agree with DeetM. I guess you play too fast. This was 10+0 time control and at move 16 you have 7 minutes left i.e. only 3 minutes used.