1. Do you think I did anything wrong and how am I supposed to play this? I can't like forget that she is an evil lich.
...
What are my odds and do you have a better idea?
The better idea is pretty much what you don't want to:
forget she is an evil lich.
So you (as a player) know she is a lich. Well you (as a character) don't. And that's exactly how you need to roleplay your character. Planning to kill her is
wrong roleplay because it's based on metagaming knowledge. I do not use the word "
wrong" lightly here, it's not about criticizing your playing style or anything like that... it's really like you're making a consequential mistake. Assuming your PCs still know nothing, you should try to proceed as usual, which might (of course!) imply being suspicious of any new NPC, but I am sure you should not be plotting a plan to kill her because that's not what you would do if you didn't know who she was.
It should also be obvious that even when players know
something from reading books in advance, they should not assume the DM uses everything by the book. Your DM might have decided that Shadowmantle is not evil, or not a lich, or maybe your adventure is set in the past and she's not
yet an evil lich... heck it could even be that the DM just read the
name, like it, and decided to use it for a completely different NPC! If you used your metagaming knowledge and kill this NPC, and then it turned out that she's a completely different and benevolent NPC, you would end up looking very stupid.
On the other hand, if the DM gives
your character some information on this NPC, then of course you are entitled and even supposed to make decisions based on that! And it is very possible that at this point your DM has in fact slipped out that info unwillingly after you made your revelation, and confirmed that she is indeed an archvillain. If that's the case, go ahead and make plans based on what your PC knows about her. That also means, if the PCs know a lich is far beyond your capabilities, then run from her, or be prepared to fail.