Tired after 12 smacks of being out of condition to me. After all, 12 is only 2 and a bit miles of walking - which ain't much regardless of whether carrying or pushing. The gym calls I think.
Re the op, I can say hand on heart my back 9 scores have improved majorly since going to a trolley. I would generally have solid fronts and fizzle off on the back 9, tipically gross +4/+3/+2 front then +7/8/9 on the back. Recently gone across to a trolley and only 5 rounds in I'm seeing the positives.
Two weeks ago shot level hc with +6/+6 hit the ball awesome but had a cold putter gross 80(par68)
Last week played with vardon, shot +4/+1 gross 77(par72)
Today shot +5/+1 for a gross 78(par72)
I should add that even though I'm on the stockier side at 34 I'm pretty fit and have never had a trolley, I hate trolleys with a passion. I caddied as a kid and always carried, played as a junior normally after caddying a round a still carried. Before the trolley I had a h2no which is a known lightweight bag, only carried 6 balls, 13 clubs and a think water proof top so far from over loaded but my scores are definitely consistent if not stronger on the back 9 since the transition