Will nostalgia be like feeling sad? or otherwise will be completely different feelings but there may be some elements that relate them, it is difficult but not entirely complex to define nostalgia, the other important thing is to know what causes nostalgia, once we feel nostalgic we behave differently than when we are not.
Searching in the trunk of memories, or thinking about that old bike from our childhood is something that gives us nostalgia, then nostalgia seems to be associated with that feeling that puts us in sadness for not being able to do the things of the past, however this may be only the beginning of nostalgia.
Time is a nostalgic element, it is very common to feel nostalgic for not being able to give back time and correct some mistakes of the past, sometimes we have our children in the present as adults and we long to see them as when they were children, but that's life, it is impossible to give back time, however that makes us nostalgic.
The past is influential net of nostalgia, it is very common to see the face of a person and clearly feel the nostalgic expression on his face when he sees photographs of his past, a past that not only involves him directly but any person who is related to affection, photographs are an element that makes nostalgia feel more latent than with memories, since we are remembering moments with the intensity that gives us the power to look at those photographs and remember moments that will never come back.
Let's take another clear example that makes us feel nostalgic, perhaps it has happened to many of you as technological innovation and the replacement of elements by more sophisticated ones makes our mind have memories of artifacts that in the present are no longer with us, such is the case of cassettes, how not to remember the good music of the 90's when cassettes still existed, however once the cd came out, it is only in our nostalgia to remember those moments.
That feeling anchored in the memories of the good longings of the past called nostalgia is not bad to feel it as long as we know how to manage it and live having nice mixed feelings of those old longings, tell me in the comments:
Is there anything from the past that you can remember and that makes you happy?
If so, we can conclude that nostalgia is not the feeling that triggers sadness, but it is the feeling that arises when remembering things from the past that can make us sad or happy, depending on how we know how to manage it emotionally.