Wales manager Jayne Ludlow admitted the failure to beat Northern Ireland cost them their European Championship dream.
Northern Ireland secured a play-off spot at the 2022 finals by finishing second in Group C to Norway, edging out Wales courtesy of their superior head-to-head record.
Wales are 21 places higher than Northern Ireland in the FIFA women's world rankings, 34th to 55th, and finished level in qualification on 14 points.
But it is Northern Ireland who progress to the play-offs with a 2-2 away draw in Wales working in their favour, and the Belfast return having finished goalless.
"Obviously those are the games that have cost us," Ludlow said after Wales ended their campaign with a 3-0 home win over Belarus while Northern Ireland were beating the Faroe Islands 5-1.
"We won the other four and the Norway games were always going to be tough.
"It was a case of where we sit, can we compete with them?
"Looking back, the home game with Northern Ireland we were especially disappointed not to come away with three points.
"Northern Ireland have a different way of playing and got what they wanted from this competition.
"They've looked at Norway in different ways to us. We believe we are building, but we still have a way to go."
Wales' goal difference was actually 13 better than Northern Ireland, with their 1-0 defeats to Norway in stark contrast to the 6-0 thrashings suffered by their rivals.
The Faroe Islands had failed to score in six group games while conceding 37 goals, but the group's bottom-placed team took the lead in Belfast to hint at a huge upset.
"I did hear something half-time (about the Faroe Islands taking the lead), that was a shock," Ludlow said.
"It was a pity they couldn't hold out for the 80-odd minutes, but well done to Northern Ireland getting to the play-offs.
"When you look at the group we've won four, drawn two and lost two.
"The good thing is we are coming closer to those high level teams in the sense that it was 1-0 twice.
"We've come so close, but we realise we can't make mistakes going forward."
