SDCard very hard to read

I would try using the SD card in other devices if possible to see if they read it. If they do, then it is likely not a card issue and instead is and issue with your 3DS.

First, you should take a flashlight or bring the device under some bright light and inspect the cartridge slot and all the contact points to see if there are any issues (bent contacts, broken contacts, anything that looks out of place).

If everything seems to look okay you can try cleaning the slots gently with a little bit of rubbing alcohol. Usually it's best to use a small q-tip but if the slot is really skinny you can always try a bit of folded up paper towel, etc.