regarding your quote from koheles, when writing that pasuk he was bitter, and not unlikely a tad depressed. I don’t think you should follow his example (ditto for the part where he drinks, parties, and spends money in excess).
Look to the end of koheles for some real wisdom- “sof dovor, hakol nishmoh, es haelokim yiroh, ve’es mitzvosav sh’mor.” As long as you’re doing that, God will be with you on your search.
On a personal note, im also in shidduchim and while not quite as melancholy about it as you are, I’ve gotta say the experience is overall really emotionally draining and difficult.! Kol tuv to you.