Working backwards:

|25.t17 26.s18 27.r16 28.r20 29.r21 30.s22 31.p17 32.q21 33.o23 34.o22 35.j19 36.i21 37.i20 38.k20 39.g21 40.f21 41.e22 42.i19 43.d20 44.e17

|23.s18 24.s20

|23.s19 24.o15 25.o13 26.s17

|23.q18 24.q20 25.u19 26.r16 27.p16 28.s21 29.v21 30.u22 31.u23 32.t17 (at his point 33.q19 would be a mistake because of 34.r18 35.p21 36.o21) 33.v17 34.u17 35.u15 36.s14 37.t12 38.r12 39.s10 40.r8 41.t8 42.s4 and now
43.r6 44.s6 45.u6 46.u3 47.v4 48.w4 49.t5 50.p5 51.o5 52.n4 53.q6 54.q5 55.p7 56.m6 57.n8 58.j6 or
43.r7 44.q5 45.p6 46.o6 47.n7 48.m7 49.l8 50.k6 51.h6 52.i5 53.g8 54.i9 55.f10 56.f13 or 55.d12 56.e9

|14.m13 was probably a simpler win. 15.m11 16.k12 17.q18 18.o12 19.r12 20.p10 21.s10 22.s7 23.t8 24.r5 25.u6 26.u2 27.u4 28.v4 or 27.w3 28.v4 29.t4 30.s3 31.v5 32.w2

White 9.L9 was strong. Black 4.G18 does not look so good in hindsight. Maybe |4.h10 would have been better.

Twixtbot agrees that |14.m13 is winning for Black. Its continuation is 15.j8 16.l11 17.f10 18.f12 19.f13 20.f8 21.h9 22.h13 23.g12 24.e10 25.i13 26.j14 27.k14 28.l15 29.q18 30.q11.

TwixtBot prefers |14.m8. It plays it out as 15.q9 16.p9 17.s10 18.r8 19.t7 20.u7 21.v8 22.s6 23.u6 24.u5 25.v4 26.v3 27.w2 28.u11 29.u9 30.q11 31.r12 32.r19 33.m14 34.n15 35.l12 36.m15 37.j13 38.n13. I'm not sure why it preferred that move.

Starting from move 15, Twixtbot finds |16.m10 to be even more devastating: 17.f10 18.f12 19.f13 20.f8 21.h9 22.h13.

|16.L12 also works for Black.

The game, as it was played, seems to get a bit dicey in those late teen, early 20s moves. TwixtBot projects a White win from this long line: |21.q18 22.q20 23.u19 24.n13 25.m12 26.h20 27.o13 28.t17 29.s19 30.r18 31.m22 32.j19 33.v16 34.s21 35.v21 36.u15 37.u14 38.u18 39.w18 40.t13 41.t12 42.s11 43.s10 44.r8 45.t8 46.s4 47.r7 48.q5 49.p8

That last line inspired me to check |23.q18 with TwixtBot. Here is what it came up with: 24.q20 25.j19 26.j21 27.k21 28.l22 29.u19 30.s21 31.v21 32.t17 33.v16 34.t18 35.w18 36.s15 37.v12 38.u14 39.w14 40.t11 41.u9 42.u8 43.v7 44.t10 45.u10 46.v6 47.s11 48.r12 49.r9 - a win for White, with some unnecessary moves on the left.

Maybe those moves on the left were not necessarily unnecessary. Comparing your last variation to mine, we get
|23.q18 24.q20 25.j19 26.j21 27.k21 28.l22 29.u19 30.r16 31.p16 32.s21 33.v21 34.u22 35.u23 36.t17 37.v16 38.v18 39.w18 40.u13 41.q19 42.r18 43.p21 44.o21 45.o23