The Ryder Cup veteran, 41, finished two over par, two shots adrift of the projected cut and down in tied 93rd.
He needed to win at least £23,905 ($30,639) - equivalent to a top-30 finish - to retain his tour place.
But the 2008 Open runner-up said: "It doesn't mean you're never going to see me again."
He later added on Twitter: "You don't get to number five in the world and help lift a few Ryder Cups and disappear.
"You come back stronger and more determined than ever."
Scotland's Martin Laird, who won the Texas Open in 2013, remains in contention at the halfway stage in San Antonio.
The 34-year-old shot a second-round 67 to leave him five under par, three shots behind the joint leaders, American pair Bud Cauley and Tony Finau.
Northern Ireland's Graeme McDowell is two under but England's Luke Donald, runner-up in last week's RBC Heritage, missed the cut on four over.
First-round leader Branden Grace carded a one-over 73 to drop back into a seven-way tie for 10th, alongside Laird.
Poulter, who shot 75 and 71, said he would still play on the PGA Tour through sponsors' invites and add more European Tour events to his schedule.
The world number 190 added he thought his struggles had been "slightly over-dramatised".
"It means I've got some work to do and I need to go away and do some work," he said.
After missing the second half of last season with a foot injury, Poulter started this season needing to earn either 218 Tour points or £271,514 ($348,000) in 10 events to retain his full playing privileges.
He has accrued 155 points and prize money of £247,335 ($317,010).
England's Ian Poulter says he will "come back stronger than ever" after losing his PGA Tour card by missing the cut at the Texas Open.