Playing Weakness cards in agreement with the rules is no trivial task. In my review i will try to explain the peculiarity of Amnesia.
When a treachery card, like Amnesia, is drawn by an investigator, that investigator must resolve its effects. Please note the wording. Some cards only let you search or look at cards from your deck, in these cases the effect does not resolve!
The effect is initiated by the keyword Revelation, which means "When a weakness card enters an investigator's hand, that investigator must immediately resolve all revelation abilities on the card as if it were just drawn."
So you follow all the steps in the rulebook under "Appendix I: Initiation Sequence".
No play restriction, cost is zero.
Step 1. No modifier applicable.
Step 2. No cost to pay.
Step 3. The card commences being played, or the effects of the ability attempt to initiate. This means the card Amnesia leaves your hand at this point.
Step 4. The effects of the ability (if not canceled in step 3) complete their initiation, and resolve. The card is regarded as played and placed in its owner's discard pile, and the ability is considered resolved simultaneously with the completion of this step.
The text of the ability states to "Choose and discard all but 1 card from your hand." There is a special rule which prevents you from discarding Weakness-cards, it says literally: "A player may not optionally choose to discard a weakness card from hand, unless a card explicitly specifies otherwise."
So if you have any Weakness card on your hand when Amnesia resolves, you have to keep it and discard all other cards!
Creating the weakness card pool
Because Amnesia is such a hefty Weakness, i would want to include a few words about the correct setup of the weakness collection.
There's a paragraph in the rulebook that states that "To select a random basic weakness, take a set of the ten basic weaknesses in this core set, shuffle those weaknesses together, and draw one at random to add to the investigator’s deck. Some Arkham Horror: The Card Game products add additional basic weakness cards to a player’s collection. Simply add these cards to the ten cards found in the core set when selecting random basic weaknesses in the future. For example: Stephanie owns two copies of the core set, one copy of the first deluxe campaign expansion, and one copy of the first Mythos pack. To create a single set of basic weaknesses, she takes all of the basic weaknesses in one core set, in one copy of the first deluxe campaign box, and in one copy of the first Mythos pack, and shuffles them together. Her basic weakness is drawn at random from this pool."
Amnesia can be two times (2x) at the most in your pool of weaknesses at the start of a campaign!
If you are new to the game, bend the rules and redraw another weakness if you play Wendy Adams.