Coach Martin Hunter was left highly critical of his players’ mentality following their three-all draw in the West Midlands.
"I'm furious", he said at full time.
"We dominate the game for 85% of the time – we're three-nil to the good and then concede three goals in the last 15 minutes. That's not good enough.
"We created numerous chances and didn't convert them. Then we needed to defend ruggedly and we didn't do it.
"[Our] two centre backs were very, very poor in the late stages of the game.
"We've scored three good goals and then it's [about] having the mental capacity and discipline to concentrate and do the simple things well. We didn't in the last 15 minutes."
Saints had led through an early own goal before second-half strikes from Jake Sinclair and substitute Ryan Seager seemed to put the victory beyond doubt.
But West Brom scored three times in the final 15 minutes to rescue a point.
The result leaves Saints four points behind third-placed Liverpool in the Barclays U21 Premier League Elite Group with two games to go, while the Merseysiders also have a game in hand.
Hunter was able to call upon the experienced Vegard Forren in defence, whilst James Ward-Prowse travelled to the Midlands on Saturday night after sitting on the bench during the First Team’s draw with Swansea.
The game couldn’t have started much better for Saints, as Lloyd Isgrove’s cross was somewhat comically turned into the net by West Brom defender DONERVORN DANIELS.
Jake Sinclair then tested goalkeeper Luke Daniels with a low strike before Jack Stephens headed over the bar.
It was largely one-way traffic for most of the first half, and Sinclair was unfortunate not to connect with a low ball across the penalty area as Saints’ continued to press.
HALF TIME: WEST BROM U21S 0-1 SAINTS U21S
Barely three minutes into the second half, Saints doubled their lead. Calum Chambers advanced from full-back and supplied the cross for SINCLAIR to strike a sublime volley into the bottom corner.
Chambers nearly got a goal himself when, after some neat footwork, the defender hit a volley from 20 yards out which hit the inside of the post.
Omar Rowe was replaced by Ryan Seager, and he was to make a near-instant impact.
17-year-old SEAGER has been scoring for fun in the Under-18s' league, and he unleashed an unstoppable left-footed strike to make it 3-0 with his first goal for the Under-21s this season.
Saints spurned several more chances to increase their lead, but at 3-0 the game looked as good as over.
Even when ALEX JONES scored a close-range effort on 74 minutes following a goal-mouth scramble, West Brom’s hopes of a comeback looked bleak.
But Saints’ lead began to look fragile once SHAUN DONNELLAN made it 3-2 with 11 minutes to go.
And in the final minute of normal time, an error of judgement from Jack Stephens allowed substitute JOEL AMBALU to steal in, go around Cody Cropper and finish into an empty net for 3-3.
FULL TIME: WEST BROM U21S 3-3 SAINTS U21S
West Brom U21s: L. Daniels, Francis, D. Daniels, C. Jones, Smart, Birch, Roofe, Allan, Atkinson (Donnellan 65), A .Jones, Nabi (Ambula 65). Subs (not used): McCalla, Lewis.
Goals: Jones (74), Donnellan (79), Ambalu (89)
Saints U21s: Cropper, Chambers, Stephens (c), Forren, Targett, Curtis, Ward-Prowse, Moore, Isgrove, Sinclair, Rowe (Seager 59). Subs (not used): Johns (GK), Turnbull, Reed, Young.
Goals: D. Daniels OG (5), Sinclair (48), Seager (63)