There are a few problems here, unfortunately.
There are three Twilight Sparkle's in this episode. Future Twilight, Present Twilight, and Past Twilight.
Future Twilight tried to warn Present Twilight about something, we never found out what it actually was. Present Twilight came to the conclusion that Future Twilight tried to tell her that she needed to stop worrying and this is what Present Twilight told Past Twilight. This might be true, but Future Twilight could have been trying to warn her of something else entirely.
If you think about it, there is an obvious problem and a loop. Future Twilight must have been told by another Future Twilight about the same damn thing, and thus a cycle or paradox of continuously telling themselves NOT TO WORRY ABOUT THE FUTURE.
This can only be fixed one way: The very first Twilight Sparkle had an actual problem, and returned to tell her past self (not the past self we saw) how to fix it but failed. Thus the endless cycle. Even though this cycle exists until one of them doesn't go back to the past, the event comes to an end for each Twilight Sparkle that gets visited by their future self (/the ending).