This weakness is quite a drawback. First any weakness that reads "When you draw shuffle it back into your deck" is cause for immediate concern. The ability to draw any weakness more than once creates that much less of an opportunity for you to draw powerful player cards. The problem is magnified when playing an event heavy deck with many one shots that mush be continually drawn. I conclude that even if the card had no other text than what I quoted above the weakness would be still somewhat of a liability.

To add to my concerns, Stars of Hyades whittles away at Sefina's greatest strength (the events underneath her character card). But the greatest problem is that Sefina takes damage (and horror) when she can't remove an event underneath her character card and with Sefina only have five starting health I see a huge a liability.

How do you over come these? Having access to these cards seems like a good start:
Alyssa Graham

Scrying

Bulletproof Vest

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 3-6 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.