Outplayed in the middlegame

Development, using open files and diagonals, minor pieces imbalances, and their strengths/weaknesses in certain types of pawn structures/centers stood out to me.

You should annotate your game. Without this, noone knows why you chose one move over another (if you even considered other moves). Show what lines you looked at and why you preferred one possible position over another (if you did that). Share your plans, how they changed or if/when they disappeared all together.

This game played out over the course of 5 weeks. I am much more experienced replying to 1.e4 than 1.d4 and did not have a clear opening in mind.  I played the opening book initially and the game went out of book with 7.Bg5

With 7...Bxc7 the idea was to castle soon. 8...Ba6 played to prevent White from castling.

After 9.Ne5 the position looked quite dangerous for me and I did not want White to play 10.Nxc6 or 10.Nxf7 and 9...Qc7 was the only move that I could see that protected against both threats.

10.Bxf6 and I have to recapture with 10...gxf6 or I am just down a piece.

11...Qa5+ tries to regain the initiative and bring out the White Queen. After 12.Qd2 12...Qb5 threatens 13...Qxb2 which is noticed and correctly defended. I no longer have an attacking threat so bring one of my rooks into the game with 13...O-O-O.

I did consider 13...Rg8 but thought 14.Rg1 and threat is averted. I was not thinking about a long term plan.

14.Nc3 I did not see before otherwise would have played 12...Qb6 instead.

14...Qa5 I am happy to trade Queen's here, I have the bishop pair and would prefer to play as Black at this point in the game.

16...Bb4 clearly I had to move the bishop somewhere. I considered 16...Bb6 but it did not look like it led to anything good for me. I did not consider Bd6 or Be7 much as they looked very passive.

18...e5 no idea of what I was thinking there.

19.b4 my bishop pair are now lacking breathing space and it looks as though I am going to lose one of them.

19...Bb5 sacs the bishop on a5 and keeps material even if 20.bxa5

20.Nc5 I think is an excellent move for a 1200 player, against most players I have played at this level I would have seen 20.bxa5 there. I think this was the turning point in the game where it all started to go wrong for me.

20...Bb6 notices how strong the knight is now and tries to trade.

21.a4 puts me in a very difficult position because there are no very good places for the bishop to go.

21...Ba6 22.Nxa6
21...Bd3 22.Nxd3

which leaves

21...Be2 22.Rde1
or

21...Bc4 22.Nxc4 22...dxc4

I went for the wrong option.

I knew that trading rooks gave White the open file, but knew that I had the option to challenge it with Rd8 and at worst case trade off the last rooks and go into an endgame with even material.

In actual fact, although material is still even White has a huge lead by this point. The White knight is much stronger than weak bishop and after 26.Nd6+ I could have and perhaps should have resigned right there.

iirc, moves 9-15 had several Queen moves and then you decided trading was the way to go (usually leads to development issues)

9...Bd6/h6 instead of Qc7 were options to consider (might be worth looking at playing both).

17...Ba5 may be a move for you to look at again

You say..."even material"

Can you describe that?

Even material just means we both have the same number of pawns, one rook and one minor piece.

Neither side has a material advantage (using the standard Reinfeld values) before 26.Nxf7, but positional factors meant White was way ahead. I had doubled pawns and a bad bishop. White's knight was able to run rampant.

I had a look at Stockfish evaluations for move 9 alternatives. It likes 9...Bd6 and 9...Qb6 best. I needn't have worried about Nxf7 too much.

"but positional factors meant White was way ahead. I had doubled pawns and a bad bishop. White's knight was able to run rampant."

9...Bd6 help with this possible issue?

Yes I think that's the main takeaway. Sometimes moving the same piece twice in the opening is the best thing that you can do.

#1
2 Bf4 and then 7 Bg5? cannot be right, black has the advantage
17...Ba5? lets the win slip away. You had to stop him from playing Nc5 and Rc1 by 17...Ba3
18...e5? lets the win slip away. You had to spoil the pawns around his king with 18...Bb5
You got the right idea 19...Bb5 but too late
21...Bc4? spoils your own paws, 21...Be2 keeps your pawn structure intact

Immediately after the game: "My opponent played well"

After analysis: "Neither of us played well"

#10 You are correct, I don't always know how to use a bishop pair plus space advantage. I think it is general inexperience, but if there are any good resources on this topic then I'd be interested in checking them out.