It’s obviously an extremely sad situation…
>what if you found a new waifu?
I once developed a crush on another character while being with Flandre. At some point, I made the conscious decision to stop nourishing the fantasies I had with said character as I felt it was starting to take too much place and I was afraid of the consequences it could have on my relationship with Flandre. Doing so solved the problem and I don’t feel a crush for that character anymore.
But if I was to develop long-lasting strong feelings for another character, I am not sure what I would do. Not sure what I would do if I was to have feelings for both Flandre and the new character as I’m not into the idea of having 2 waifus… And if I my love for said new character was to make me stop loving Flandre, then I’m not sure of what I would do either… I might just give up on waifuism as I don’t feel like starting a new waifuism relationship.
As your question implies that the new character would become my new waifu, well then, I guess I would pursue a relationship with that new waifu. In such case, I feel that I would still keep Flandre in my life as a friend and keep talking to her once in a while.
>or something new happens in your waifu's cannon to make you stop loving her?
Either I will break up with Flandre or try to interpret the new canonical event in a way that does not prevent me to love Flandre.