After 10...Ne4, continue development with 11.Re1, or improve your position with 11.Bd3. The move 11...Be1 wastes a tempo and takes away a sport for the rook on the open file. You are not worried about the bishop pair if you can get rid of his most active knight in the center of the board with an inactive bishop not doing anything. 13.c4? weakens your queenside pawn structure and the c2 pawn can no longer be used to protect the d4 pawn. After 13.c4 Black could have played 13...Qb6, attacking two of your pawns, at b2 and d4. Black can then plat ...dxc4 and isolate your queen pawn. Better may have been 13.Bd3 to place your bishop on the diagonal with the most space and threaening his kingside. After his 17...a5, 18.Bd2 is not the best square for your bishop as it blocks the square from the knight if Black had played 18...a4. Your knight would then have to retreat to 19.Nc1, and after 19...dxc4, Black threatens the unproteced d4 pawn and leaves you with an isolated queen pawn. After 18...b6, 19.a4 doesn't do too much (...a4 Nc1 is OK for you) compared to getting your rooks activated (Rae1 or Rfe1). After 23.Nxe6 fxe6, you missed winning a pawn after 24.Qxc6. Be greedy and take the pawn and win the endgame. After 25...Rac8, it was better to take with queen instead of bishop. 26.Qxc6 Qxc6 27.Rxc6 threatens to win another pawn with 28.Bxa5. After 30.Rxc8 Rxc8, you missed 31.Rxe4, winning a pawn since 31...dxe4 is impossible since Black cannot move into check. After 34...Ndf6, 35.Be5 is not that strong either. BEst seems 35.Qb5, threatening 36.Qxa5. After 35...Qf5, instead of 36.Bxe4?!, play 36.Be3 (to prevent ...Qf2+) and threaten 37.Rf1. After 36...Nxe4, perhaps 37.Bf4 is better instead of 37.Be3 since Black can play ...Rf8 with control over the f-file and a nice threat of ...Ng5, then ...Nh3+. After 39...h6, 40.Re1 is too slow and you want that rook on the open f-file. Your are just forcing where Black wants to put his queen on f5 anyway, threatening ...Ng4. Try 40.Qd2 and mobilzie your queen with the maximum space to attack. After 40...Qf5, perhaps 41.Re5 or 41.Rf1 and play it safe. I don't think 42.Rf1 loses immediately, but you had a better move with 42.Kg2, and if 42...Rf8, then 43.Bg1. If 42...Rc2, then 43.Re2. After 42...Rc2, insterad of 43.Be1, you could try 43.Qf4 and exchange queens. 43...Qxf4 44.gxf4 Rxb2 45.Be1, threatening 46.Bxa5. 44.Rf2 looks like the losing move. You had to try 44.Bf2 Rxb2 45.Qd3, and if 45...Nxf2 46.Rxf2 Rxf2 47.Kxf2 Qxh2+ 48.Kf3, you may be able to draw this queen and pawn ending.
How to lose an unlosable position
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As I commented earlier, I am beginning to analyze a game a week, because I want to become a better chess player. Here is a game that I felt I played superbly the first 34 moves and then let it go. I missed an obvious tactic that I saw as soon as I analyzed the game (without the computer). Also, I made some horrible decisions near the end, where I still was up a pawn. Please feel free to comment, as I want to improve.