Wistful
Etymology
A somewhat murky formation, probably blending the old word "wistly" (meaning intently or longingly, as in gazing) with "wishful." The pull of "wish" is clearly felt in the meaning. The result names a mood of yearning attention — longing looked at through a veil of soft regret.
adjective · /WIST-ful/
Full of gentle, yearning longing, usually tinged with faint sadness.
What it names
The soft ache of looking back at something loved and lost, or forward at something quietly wished for.
Example
She grew wistful looking at the old photographs, half smiling at summers that would not return.
Related idea
Nostalgic is longing specifically for the past; wistful is broader, a gentle yearning that can reach toward the future or the merely wished-for as easily as the remembered.