The Hawks were left to devastated as they agonisingly missed out on making the Skrill South play-offs on goal difference.
A win at already relegated Tonbridge Angels would have guaranteed them fifth spot.
But all they could manage was a 0-0 draw at Longmead Stadium.
That allowed Dover in to take the final place.
Their match at Hayes & Yeading finished late, so news of their last-gasp win only filtered through to the Hawks after a 10-minute delay.
Manager Lee Bradbury was distraught to see his battle-weary troops suffer such a painful end to the season.
He said: ‘It can be a cruel game sometimes.
‘The 10 minutes stood on the pitch waiting to find out the result of the other game were horrible.
‘I can’t describe the feeling when we heard Dover had grabbed a late winner.
‘We are all devastated and the players are out on their feet.
‘They have to be congratulated, though, for the efforts they have put in – particularly since the start of the new year.
‘The number of games they have had to play in the final two months has been staggering.
‘The players have given everything but the final two games were probably a bridge too far.
‘I am convinced if we had played the games over a normal season we would have made the play-offs.’
The visitors realised from the start that Tonbridge were not going to roll over for them.
Beaten 7-1 in their previous game the Angels were determined to regain some personal pride.
The Hawks were going to have to work for it and, after the opening 10 minutes, they started to impose themselves.
Perry Ryan thought he had made the all-important breakthrough, heading in a 17th-minute free-kick.
But his celebrations were cut short by the offside flag.
The best chance, however, came 10 minutes before the break.
Winger Christian Nanetti embarked on a mazy run into the area, before he was tripped.
The referee immediately pointed to the spot.
But Donnelly’s penalty was well saved by Lewis Carey, diving to his right to push the ball away.
The Hawks put their disappointment behind them and pushed for long spells in the second period without seriously troubling Carey.
Ciardini had another effort disallowed for offside.
But despite not finding a way through, it still looked as if the Hawks had done enough to sneak into the play-offs.
Then came the morale-sapping news of Dover’s victory at Hayes & Yeading.
Bradbury felt marginal decisions went against his team on the day as they were left frustrated by failing to score.
He said: ‘The offside decisions were very tight and we felt we could have had another penalty in the second half.’
‘It is frustrating to come away with nothing after everything we have put into it.’
Hawks: Buchel, Newton, Blanchett, Donnelly (Kabba, 79mins), Strugnell, Woodford, Ciardini, Atangana, Nanetti, Ryan (Hutchinson, 89mins), Oli.