A weakness where the worst element is that it recurs and recurs and recurs. Looking at the two elements, the effect and the discard condition, we get:
The effect: This is a fairly complicated weakness with 3 parts.
First, you need to discard one of the 5 special events under Sefina. Losing one of her events can be trivial or damaging, depending on the event and situation. It's pretty common to have a card or two under her that aren't useful in that scenario or that you've used up your The Painted World and don't have a free action to get them into your hand; that doesn't hurt so much. Other times, you were counting on that Deny Existence or Slip Away and now what's the plan? Sometimes, you've drawn a lot and pulled all the cards out, so no discard!
Second, if you can't discard, you take damage and horror, which is pretty punishing for a 5 Health investigator. Under normal circumstances, this isn't going to happen until you've drawn the weakness #-6 times, but that can happen. and aren't all that great with soak, so Sefina doesn't have great options -- maybe Leather Jacket or Robes of Endless Night (2) and Lonnie Ritter?
Third, you shuffle it back into your deck unless that deck is very thin. Without this, this wouldn't even be average badness, but here we are. There is very little mitigation -- she can take the basic Scroll of Secrets and hope to snipe it off the bottom of the deck. Quantum Flux can make your odds better.
All of these are very situational; draw this once, and it's no big deal; draw it 5-6 times, and it will wreck your day and probably the scenario
The discard condition: You kind of don't, except just before decking yourself.
This is a hard weakness to rate because it's so situational; everything considered, it's above average, especially if you have a draw-heavy deck or bad luck.