One continuing frustration of the Vietnam veteran is trying to explain how it really was to those of us who watched the war on TV.

The sit-trag series was canceled with our pullout in the spring of 1975, and, apart from fragmentary "retrospective" reruns, we've been able to forget about it. If you wish to continue in that placid state of mind, stay away from the Washington Project for the Arts Gallery until after the end of July.

Betweeen now and then the gallery will be filled with the works of two-dozen members of the Vietnam Veterans Art Group. The range is from art to agony, and the finest pieces embrace both, such as: ABANDONED by John McManus, a sculpture in stone and metal of a cowled, cowering, mutilated -- child? woman? man? PIETA by Mike Page, a wood sculpture of a Marine cradling a dead baby. ARTIFACT III FOR KING by Brian Maxfield, in which by covering a blown-away corpse he makes the horror all the more manifest. The only qualification for entering works in the show was having served in Nam, but the unevenness of the artists' abilities makes the assemblage all the more affecting. And the works that fail allow time to catch one's breath between body blows. There is an endless round of some 500 slides that show, all helter-skelter, the naive pride and courage of the young soldier, the warmth of camaraderie, the beauty of the country and its people, the crumminess of war-strewn landscape, the boredom and fear of service, and some of the searing reality that got edited out of the nightly news: the bodies from which those body counts came. But for all the horror it shows, the exhibition is not a horror show. It is a terribly honest effort, by men who used to be young, to find the truth and meaning of the tragedy of Vietnam. The truth of it may be, judging from this testimony, that the tragedy was the meaning. VIETNAM: REFLEXES AND REFLECTIONS -- Through July 31 at the Washington Project for the Arts, 404 7th Street NW. Open from 11 to 5, Tuesday through Saturday. Further exhibits will be on view June 20 to 24 in the Russell Office Building and September 12 to 23 in the Cannon Office Building.