Thank you Jeff for pointing it out! (I thought I put some words in front of when, but it seems like anything can happen at night, especially for me. Maybe the website cleared its cache all by itself and my language structure was gone as well. :()

Perfect use of “come together.” You can say, “That was when I realized academics were not for me.”

Perfect!

I would put this in the future tense: “this trip will only come together when I buy tickets and book a hotel.”

That’s great – you’d just want to say, “…came together in a comprehensible…”

This is good – you would not say, “diet frequently” since diet is more like a continual action, whereas “exercise” can be considered a one-time thing you can do over and over. You can fix this by just changing the order of the words: “…if you diet and exercise frequently, then it will come together…” and in that case, the word “frequently” simply modifies the word “exercise” and not diet.