Can you help me improve?

This is the game I played against computer. I just can't beat it. This is one of the openings I use.

What I wonder is what the main reason for my losing is. Is it blunders or is it some flawed tactics. I do not have a lot of experience in chess. So, I wonder what I should mostly focus on.

Ive played over the game. I am a decent chess player. Elo rating of about 1900, 2000 on a good day.

Your opening knowledge is more then good enough for your playing level. No need to focus on that.

I would train yourself on tactics/stability 18 c4 instead of f4 to keep the unneccesary loss of a pawn and centre control. There are many cases of you not seeing basic tactics. This game is not lost by huge blunders but many small mistakes.

If you would study tactics and play more regularly you would gradually improve your game. Also take your time during your matches if you can. I noticed this first hand yesterday when i lost an important match when i still had 30m on my clock.

Go over your games afterwards. espessially your losses. play them with an engine or just a chessboard. You will notice what went wrong in your games.

just me looking at it, with no computer, I see two things that let the machine get a leg up against you.  

11) b5 is a temporary threat that is easily countered and leaves your pawn deep in black's territory and undefended.  It could maybe be used to do something, but you more or less put it there to harass his knight then abandoned it.    This looks like a good move to have castled instead.  Or, if you play b5, come up with a plan to defend it or get something for it, perhaps follow with a4? 

and 15) Bc4 traps your knight, it has NO escape squares.  The knight that is in a nice place, sorta, becomes a liability.  I think no matter what you do it will cost you at least a pawn, at that point (I could be wrong, again, I am not a 20 depth computer but just an average player).

Those two moves sort of gave black just enough to get the initiative and pressure on you, and you never really recover from it. 

Usually against a good computer, down a pawn with weakness is enough to lose.

At 18...Qxb5 White's down a pawn, weak on the light squares, worse pawn structure and exposed king.

24. Qc3 drops another pawn and I didn't notice any compensation. Now Black's 2 pawns up with a clear Qside majority and it goes downhill pretty fast from there.

Exchanging with advantage by move 35...Ke6 Black has four passed pawns, total dominance on the light squares (making White's B worth less than a full piece) and better King position.

It might be worth noting at game end, Black's got 7 of his 8 pieces on the light squares.

To draw with a good computer, being aware of all weaknesses, trying to exchange only with advantage and making sure that there is sufficient compensation for a pawn drop might be the best way to go. Even then and if one can be slightly better, computers are great at finding perpetuals. 

not blundering should be your main focus. I recommend the free system by Chuzhakin. By going over a list, you will make less blunders and get better at finding out what to look for.

It is a system of finding hazardous elements and using those to choose moves. If the opponent has no hazardous elements, then you seek to create them.

Analyzing master games like Capablanca with the Chuzhakin system makes you understand the games much better... and of course your own.