Jo Hursey, a former staff sergeant from Royal Wootton Bassett, wants people to support the Help for Heroes' message wall initiative, which received thousands of messages last year.
She said: "Veterans can sometimes feel they are forgotten by society.
"And if they're living with physical injuries, disabilities or mental illness they can become isolated - and especially at Christmas, their mental health can spiral down even further."
Jo, diagnosed with PTSD after serving in Northern Ireland, is familiar with these feelings of isolation.
She added: "These days, I could be surrounded by a thousand people but still feel very alone."
Jo, who now lives with depression, anxiety, and a hearing impairment, explained the importance of the public's support to those in the armed forces.
The charity aims to send over 10,000 Christmas cards to veterans including messages of support.
The Office for Veterans Affairs and Family Survey, published in June, shows that nearly one in three UK veterans experience loneliness.
Last winter, requests for support to Help for Heroes were 25 per cent above average.
Latest data shows there are more than 40,000 veterans living in Wiltshire.
Anyone wishing to add to this year's message wall can do so at sendamessage.helpforheroes.org.uk.
Jo added: "It's amazing how a few heartfelt words can remind you that you're seen, valued, and appreciated.
"Because no-one who served their country should feel alone, forgotten, or left behind this Christmas."