Kent head coach Matt Walker bemoaned his side's missed opportunities in their final Vitality Blast game of the summer.
Spitfires were well placed to defeat Glamorgan on Sunday at Canterbury but let the visitors recover from 95-5 to double their score in 6.3 overs.
Kent then lost early wickets in the powerplay and never recovered from 17-3, despite some late hitting from Darren Stevens and Grant Stewart.
It left Spitfires rock bottom of the South Group with just three wins - equalling their worst-ever T20 campaign just a year after winning the title.
Walker said: “We finished how we started really and a lot in between.
"It’s pretty obvious where we lost this game. We were brilliant up to 13 and a half overs, I thought we were outstanding with the ball and were really back to our best, especially our seamers.
"I thought we bowled really well and 150 was about right, but those last seven overs cost us 100 runs and some of it was our own doing. Some of it wasn’t but there were missed opportunities and missed catches, which is never a good thing when you’re trying to squeeze a side.
“We were really only a couple of wickets away from getting into their tail. We gave them an opportunity to make us pay and they did.
"When you lose three wickets in the powerplay it’s very difficult to wrestle it back from that situation.
"There was a bit of huff and puff and some decent cameos but nothing of substance, which you need when you're chasing 190.
“We had to gamble a bit and people had to chase down 12, 13 an over and we came well short.
"We’re extremely disappointed with how today went and how the whole campaign went. It’s not what I’d hoped for and not what I’d expect.
"We know this division’s very difficult, we know you’ve got to play really good cricket to get through the qualifying stages and if you miss out having played good cricket you accept that, but I don’t think we’ve really played our best cricket.
"On a few occasions we did against some good sides, but overall we’ve got a lot to talk about and a lot to think about.”
Glamorgan chose to bat and were 46-0 at the end of the powerplay, but lost Tom Bevan for 21, lbw to Qais Ahmad attempting to reverse sweep, before Matt Milnes bowled Sam Northeast for 33.
Qais (2-36) switched to the Nackington Road End and immediately bowled Chris Cooke for one and Colin Ingram was out for 26 when Grant Stewart sent his off stump flying.
Jack Leaning then had Eddie Byrom caught by George Linde at backward point for a second ball duck and Glamorgan were 95-5.
Then Dan Douthwaite and Billy Root turned the game on its head, although Kent were not without their chances to break the partnership.
There were two costly drops in Milnes’ 17th over. Root skied one to Tawanda Muyeye before Douthwaite was spilled by Jack Leaning.
The penultimate over from Milnes went for 22. Root hit the first ball for six and was then caught by Muyeye off a no-ball. A further no ball and five wides gave added impetus to the innings, with Kent looking increasingly sloppy before Fred Klaassen’s final over went for 19.
Kent’s chase got off to an equally dismal start when Joe Denly was run out by Root in the first over.
Muyeye cracked three elegant boundaries but then swiped Jamie McIlroy (3-31) to Byrom and was out for 13, before Jordan Cox fell for one, pulling James Weighell to Ingram at mid-wicket.
Alex Blake hit his 100th Blast six when he hammered Douthwaite over cow corner, but when Leaning chipped a return catch to Andrew Salter for 10 Kent, were 46 for four.
Stevens’ return after a lengthy injury lay-off produced the biggest cheer of the night but this was a rescue act beyond even him, although he did flick Prem Sisodiya for a six that nearly cleared the flats on the Old Dover Road side.
Blake went down swinging for 32, skying a Salter delivery to Northeast and Stevens top scored with 34 before he hit a Douthwaite full toss to Bevan on the boundary.
Stewart offered some fight with 23 from 10 before he hit Sisodiya to Bevan and Qais was bowled by McIlroy for four. McIlroy then had Linde caught by Root for 21 and Glamorgan’s victory was sealed when Milnes hit Douthwaite to long off and Bevan held his third catch of the innings.
The death overs proved fatal for Kent, as an unbeaten stand of 95 between turned the game on its head, steering Glamorgan to 190 for five. The visitors had been 95 for five in the 14th over, but Douthwaite smashed 51 not out in and Root ended unbeaten on 38.
Qais Ahmad took two for 36, but dropped catches and indifferent bowling that gifted the visitors 20 extras meant the total was at least 30 runs higher than it could have been.