I keep missing important moves/captures/checkmates

I just played this game as White and during the analysis it revealed that I missed 4 checkmates.  You can see where I was going with the queen and bishop working together but I just couldn't see the mate. I was going to try for a back rank mate with the two rooks at the end.

I also missed an opportunity to grab Blacks queen on move 15. This is again something I've missed twice in my past few games. 

This is actually my best win, though it was won game abandoned (my highest rated opponent so far). 

It's certainly one area I need to improve on. 

How was I doing in this game compared to my 900+ rated opponent? (I began the game rated 776.) 

I'm taking away the positive that at least it can be seen that I was doing something, there was an idea there but I just didn't have it all in hand. And the fact I made no blunders. 

Note to self: LOOK AT THE WHOLE BOARD! 

ETA: I've just noticed that 20. Rf8# would've been mate rather than 20. Rae1.  Drat, drat and double drat! 

I recommend you doing a lot of puzzles. It really helps. And don't do the 4 knights Italian, I suggest you try the 4 knights Spanish variation.

I have to agree that doing a lot of tactical puzzles will help. There are so many standard mating patterns that you have to internalize. For instance, 10 Qxf7# is one of the classics that would leap out like a laser beam to a more experienced player. And, again, 13 Qf7# is the same thing. It's not that you're a weak player with no potential, it's that you haven't yet formed an arsenal of patterns that you see instantly. This takes time, but everything worth doing does take time.

I also recommend that you start playing over the games of some great players, starting with Morphy. His games are freely available here or on chessgames.com. Good luck.

Hi! My name is Lauren Goodkind and I’m a chess coach based and chess book author based in California. 

I have tips to help you improve your chess skills so you can win more games.  

I recommend playing with a slow time control, such as game in 30 minutes.  You need time to think.  Beginners tend to make a lot of silly moves with very little time.  This makes sense since there’s a lot of pieces on the board.  
 

I also offer a  free beginner’s free eBook on my website, www.ChessByLauren.com in case you are interested. The book is about asking questions before each move.  
   Before each move, I highly encourage you ask questions before every move such as, “If I move here, is it safe?”, “Can I safely capture a piece?”, and more.  

Also consider all checks and captures on your side and also your opponent’s side. This will help you not miss any checkmates in one.   If you are past this stage, then find a forcing winning line.  

Learn basic tactics such as the fork, discovered attack, pin, and more.  I offer interactive puzzles on my website: https://www.chessbylauren.com/two-choice-puzzles.php  
If you are serious about chess, I highly recommend you hiring a chess coach to help you.  
I hope that this helps.  

Lessons, puzzles, and practice against computers (or live people). It takes some practice with chess visuals to see things. After a while, you'll start to see more moves and plan plays better.

After looking at the game I think wasn’t multiple different missed mates. It was the (roughly) same mate multiple times. At least two were similar, one was different but in principle the same. These kind of mates should be easy to find  at all ratings,  which makes it difficult  to give constructive feedback.

It’s not clear in your post if you understand what you missed. Because the underlying reason might change what you need to work on.

Mating chance on move 10.Did you play to fast? If you move to check, before you find to the move to mate, then you need to chill and look at all the options. Or did you make the conscious decision to play the smaller piece rather than the big piece because it felt safer? In that case you need to study mating patterns more. My question is, did you consider both options and chose poorly or did you not even consider the mating queen move?

Mating chance on move 13. You get the exact same mating opportunity. What is the reason you made such an unintuitive move? Time pressure? I would like to know if you, in hindsight, consider this a difficult mate to find.

In between you miss the opportunity to take a free queen and a free rook. Did you make the wrong decision or did you move before considering all options?

Mating chance on move 19. Same question: why? The good news is, this is also easily trained by doing puzzles.  

The main question is whether you need to work on your understanding of the game or you need to work on your focus of the game

Puzzles: yes, yes, yes!

About the opening recommendation: it does not matter.

well use the full 30 minutes to its full potential. before playing a move ask yourself if it is obviously bad. if yes, dont play it. and solve untimed puzzles as they can help immensely. timed puzzles might give a rush which will make you solve them badly. and most importantly, have fun!

On this site there is a puzzle game called solo chess where you have to condense everything to one piece.

Thanks for the responses and tips guys. 

Duckfest, I believe the problem was I was looking at one move and thinking that's a good move, without considering other options. 

I will take on board what's been said and try to improve on these points.