Add Gen Xers to the long list of Americans who fear they won't have a sizable enough nest egg to retire.

Nearly four out of 10 (37%) of Generation X — those born between 1965 and the late 1970s — say they would like to stop working for good and "fully retire" someday, "but will not be able to afford to," a new survey from TD Ameritrade, an online broker based in Omaha, found.

Other gloomy Gen X retirement findings:

43% say "they are behind" in their savings.

Half (49%) are "worried about running out of money" once they leave the workforce.

Nearly two out of 10 (17%) say they "aren't saving or investing for anything."

Only a third expect to be "very secure" in retirement — vs. nearly half of Baby Boomers.