Is there a word for being sad about knowing that the things that make you happy will eventually go away

Solution 1:

May not be an exact fit but perhaps you can use wistful (optionally with already). (Your usage of kinda in the example also conveniently allows for an approximate expression.)

"OK I guess, feeling kinda wistful (already), I wish this holiday could go on forever."

M-W:

wistful adjective

: having or showing sad thoughts and feelings about something that you want to have or do and especially about something that made you happy in the past

She was wistful for a moment, then asked, “Do you remember the old playground?”.

Solution 2:

I am not sure this is a perfect fit, but Sunday Night Blues refers to the feeling you get on a Sunday evening thinking that tomorrow is a working day.

Solution 3:

I'd use bittersweet in your example. It doesn't contain the whole sentiment of knowing what makes you happy will go away though.

If not referring to an actual plant (vine), bittersweet is defined as

"pleasure alloyed (mixed) with pain". (Merriam-Webster)