I'm not sure how to use this site's chess diagrams
First off, to learn how to make chess diagrams here read carefully answers to this post.
As for your question, I will give you brief comments about the game first, and then analyze the endgame.
Also, any comments on my play/computer's play would be highly appreciated.
Since SO hasn't added written commentaries to follow a move, I will refer to the move, and then comments will follow.
If you have further questions leave a comment.
Let us start:
Your opening play was not precise, but you still had a playable position until
15.e4. After that move Black had a small advantage and an initiative. I do not play
1.e4, nor do I play the French defense. Even if I did, I would advise you to prepare your openings well by consulting an opening book. That should remedy the mistakes you made in your game.
Although Black was better, you still could count on a draw.
21.a3 seems desperate-it looks like you had no idea what to do, and it is a pointless move. You have no advantage at the queenside so there is no reason to start action there. This would also be slow plan and would weaken you more than helping you. At this moment it was crucial to somehow break the pin. One idea would be
b4-something like this:
[Black "Computer player"]
[fen "4r1k1/pp2q1pp/2pb4/3p4/4P3/2P2QP1/PP4BP/4R1K1 w - - 0 1"]
1.b4 a6 2.Rd1! dxe4 3.Qxe4! Qxe4 4.Bxe4 Rxe4 5.Rxd6
with some chances to draw.
23. you had a chance to execute my combination and get back into the game. At the end of the combination the opponent's king would be threatened by mate, so you could get a slight advantage there.
After missing this chance, you had to struggle for a draw-I would try to reorganize my pieces like this:
[Title "Suggestion for piece placement"]
[fen "4r1k1/pp2q1pp/2pb4/3p4/4P3/2P2BP1/PP2R1KP/4Q3 w - - 0 1"]
so I can achieve
exd5 and equalize.
The rest of the game the computer played pathetically and squandered the advantage.
Is this endgame winnable(for White)?
As for the endgame, it is a draw-Black plays
Bf4 and enforces blockade on the dark squares after which White has nothing. It will look something like this:
[Title "Endgame analysis"]
[fen "8/6p1/1p3k2/1P2bP2/6PK/3p1B2/8/8 w - - 0 1"]
1.Be4 d2 2.g5+ Ke7 3.Bc2 Bf4 4.Kh5 Kf8 5.Kg6 Be5 6.f6 gxf6 7.gxf6 Bd4 8.f7 Bc3=