I have about 10 credit cards which I ended up not using for various reasons and I feel bad for having to throw them away in the trash, this is the case for many other things, but seems quite inefficient to just cut them up and throw them... Surely there must be a better way to recycle/re-use them?
Before doing anything make sure your credit cards are not usable/active anymore. You don't want anyone to misuse them!
Reuse is almost always preferred over recycling because that usually doesn't require new resources. One reuse option is to cut your cards into guitar picks. Several other reuse options are listed here but I'm sure there are plenty of others.
This website says that
the first thing you should do is verify whether or not your municipality accepts number 3 plastic, and specifically credit cards, membership cards, or gift cards. If they do, play it safe and cut off any holograms or chips, which should go in the trash
I recommend to also make sure the magnetic strip isn't readable anymore if your card has one. Better safe than sorry.
Unfortunately the chance that your municipality will accept credit cards is low. The same site explains that recycling credit cards is hard because
cards these days contain all kinds of technology that interfere with the recycling process......[for example] contamination problems from holograms, scratch-off material, and embedded chips