My usual way to play the opening would have been

|7.l16 8.m12 9.h12 10.j8 but I felt this would have played into my opponent's hands. So instead I tried a more direct attack. Something about the loose connection between black's n14 and q19 pegs looked exploitable. This is fancy talk from someone who lost the game, but that's one reason why I post here, to see what if anything was wrong with my judgement.

I believe my |33.j7 threw away my carefully cultivated advantage. I needed to play |33.s9 instead, to see how my opponent reacted, before I played my pincer attack move. For example, after

|33.s9 34.r5 35.q8 36.p6 37.o8 38.o7 39.i9 40.j8 41.j7 42.g11 43.k9 44.k11 45.n6 46.p9 47.r6 OR

|33.s9 34.r5 35.q8 36.p6 37.o8 38.o7 39.i9 40.h8 41.k8 OR

|33.s9 34.r5 35.q8 36.p6 37.o8 38.o9 39.m9 40.l9 41.k8 42.p7 43.h11 OR

|33.s9 34.r5 35.q8 36.p6 37.o8 38.o9 39.m9 40.p7 41.h11 42.h8 43.j10 44.k8 45.l7 46.l10 47.i8 48.j7 49.j8 50.j9 51.k7 52.l6 53.m5 54.m6 55.l9 56.n6 57.n3 58.m8 59.m7 OR

|33.s9 34.r5 35.q8 36.p6 37.o8 38.m7 39.k8

I think 18.m12 was a perfect move:

19.l12 20.o11 21.g19 22.m10 23.g10 24.f13 25.f12 26.i19 27.p20 28.m16 OR
19.l12 20.o11 21.p10 22.i19 23.n20 24.m16 25.n16 26.l18 27.t10 28.p13 29.t17 30.r14 31.t13 32.s13 33.r12 34.q12 35.v14 36.s11

after 18.m12 i was sure i had a winning position, but 28.r14 was unnecessary..

28.s9 29.u8 30.t6 31.q8 32.r11 33.r12 34.r7 35.q10 36.o8 OR
28.s7 29.q8 30.p9 31.s9 32.q7 33.t5 34.r5 35.u8 36.v5

but after |31.t15, 32.s7 still working..

32.s7 33.q8 34.p9 35.s9 36.q7 37.t5 38.r5 39.t7 40.v5 OR
32.s7 33.q8 34.p9 35.u9 36.r5 37.u6 38.u4 39.p6 40.r8 41.t8 42.t9

32.s8 was big mistake..

37.p6 38.q9 39.t7 40.t4 41.r8 42.r7 43.r6 44.p4 45.p5 46.o6 47.n7 OR
37.t7 38.t4 39.p6 40.o4 41.q4 42.q3 43.n5 44.q9 45.r6 46.m5 47.l6 OR

i was very lucky because David made mistake and played 37.q8.

|28.s9 29.u8 30.t6 31.t7 32.v7 33.r8 34.r11 35.p12 36.p10 37.n8 38.o8 39.h11 OR

|28.s9 29.u8 30.t6 31.t7 32.v7 33.r8 34.q8 35.p7 36.t8 37.r10 38.r12 39.u13 40.u10 41.n8 42.q10 43.h11 looks like a win for black.

|28.s7 29.s8 30.t9 31.r10 32.q8 33.n8 34.o8 35.p7 36.p10 37.h11 OR

|28.s7 29.s8 30.r5 31.j8 32.k11 33.h9 34.g9 35.q7 36.o6 37.n6 38.n4 39.o8

After |31.t15 I also had another way to win:

|32.s7 33.m8 34.o8 35.o7 36.p6 37.h11 38.h8 39.j10 40.k9 41.l9 42.l7 43.l10 OR

|32.s7 33.m8 34.l9 35.o9 36.q11 37.r9 38.r10 39.t8

IF this is all correct, it means I was winning from the opening, and threw it away with |37.q8

I am amazed I missed the winning sequence at the end, thanks for showing me!

Uh, I should have said "looks like a win for white" sorry.

what about

28.s7 29.s8 30.t9 31.r10 32.q8 33.n8 34.m8 35.l7 36.o9 37.h11 38.g11 39.j10 40.k9
OR
32.s7 33.m8 34.n6 35.o9 36.k7 37.p7 38.r9 39.h11 40.h8 41.j10 42.l9
OR
32.s7 33.m8 34.n6 35.o9 36.k7 37.q8 38.p5 39.j7 40.j5 41.l8 42.h6

im not sure what about that..
|32.s7 33.m8 34.n6 35.k7 36.i6
|32.s7 33.m8 34.n6 35.k7 36.i6 37.o9 38.r9 39.h11 40.j8 ??
|32.s7 33.m8 34.n6 35.k7 36.i6 37.s8 38.r5 39.q7 40.o4 41.o9 42.k5 ??

OR
|28.s7 29.s8 30.t9 31.r10 32.q8 33.n8 34.m8 35.l7 36.o9 37.h11 38.g11 39.j10 40.k9

OR
|28.s9 29.u8 30.t6 31.t7 32.v7 33.r8 34.q8 35.p7 36.r11 37.r12 38.u10

|33.m8 34.n6 35.o9 36.k7 37.q8 38.r10 39.h11 40.h8 41.j10 42.l9

|28.s9 I cannot find an answer to. So your m12 was a winning move after all.

I still have a line against |28.s7:

|28.s7 29.s8 30.t9 31.r10 32.q8 33.m8 34.o8 35.o7 36.p6 37.h11 38.h8 39.j10 40.k9 41.l9 42.l7 43.l10 OR

|28.s7 29.s8 30.t9 31.r10 32.q8 33.m8 34.n6 35.k9 36.k11 37.o9 38.r12 39.u13 40.q10 41.p9

So I look earlier in the game for my losing move. Maybe I should have played |17.h12 instead of i13.