I want to make the bowstring bend backward like this:
Bow:
Bow with the string pulled back: (I want the top and bottom parts of the string to curve inward like this)
I want one face of the top and bottom parts of the string to go into the wooden part of the bow and the opposite faces to go into the middle part of the string.
I would put some attachments on the middle part of the string at the top and bottom, and then some more attachments on the top and bottom of the bow.
Then you could either use a RopeConstraint | Roblox Creator Documentation to visualize the connections automatically, or use CFrame.lookAt to position the two parts yourself. Size the parts with (b-a).Magnitude, where a and b are the world positions of the two attachments you’re stretching the part between.
Requires a bit more work if everything’s welded together, but you didn’t mention that 
RopeConstraint would look pretty cool to be fair.
