Northants went down to a comprehensive 59-run defeat, but Sadler felt it was a match that could have easily gone his side's way, but for costly lapses in concentration with bat and ball.
First, the Steelbacks allowed the visitors to rattle up 48 runs in their final five overs to post an imposing 295 for six, with Joe Cooke hammering 33 not out from 18 balls and Tom Cullen 58 not from 44 as Glamorgan rallied from 227 for six.
Then, with the bat, despite half-centuries from Ben Curran and Tom Taylor, wickets were lost at regular intervals with the highest partnership the 65 put on for the fourth wicket by Curran and Saif Zaib, who made 41.
It meant the Steelbacks couldn't get any momentum in their innings as they were bowled out for 236 with 5.4 overs remaining, the impressive Taylor left stranded on an unbeaten 65 from 59 balls.
“Glamorgan built their innings really well," said Sadler, who is leading the 50-over team for the next month in the absence of David Ripley who is working with London Spirit in The Hundred.
"They had a little bit of a wobble, we got on top and then for about 45 overs of the first innings we were magnificent.
"Then we didn't quite execute those last five overs.
"Obviously, that puts a different perspective on the game when we're chasing 290 instead of 260, 270 maybe.
“Then batting wise, we lost a couple of wickets up top, which can happen in this format.
"I thought we played pretty well and then we had the game by the scruff of the neck and then lost a couple of soft dismissals and that kind of put on the back foot.
"So, there are some good points to take from this, but ultimately, it's a feeling of disappointment."
The highlight of the performance from a Steelbacks perspective was the effort of Taylor, who claimed two for 54 as well as hitting 65 not out.
And Sadler believes there is more to come in terms of batting from the former Leicestershire all-rounder.
“Tom Taylor is batting very low at seven, we know he could bat higher and in time I’m sure he'll find himself further up," he said.
"But we went for the extra batter.
"It seemed to have worked at halftime because we were in a position to go knock those off and I was confident we would.
"So yeah, Tom's a fine cricketer. He’s going to get better and better, the more experienced he gets. But unfortunately, he ran out partners in this match.”
The Steelbacks return to One Day Cup action on Wednesday when they travel to Scarborough to take on Yorkshire (start 11am).
The White Rose have won one and lost one so far in the competition so far, with their victory coming on Sunday when they beat Leicestershire by seven wickets.