Can you please give more details, at least the address? The Bitcoin transactions can be easily tracked.
Also the type of wallet you used could help.
You should have kept the wallet file though, it would have made the things much easier...
This is the address where the rest of my coins went to 18rnXzMAzrD5wiR4Go91TPNJWVhhnnxnrh. Also I am using Bitcoin Core. Is there anything I can do because my wallet was encrypted and I never spent the rest of my coins
SOOO... basically, you got one input of 0.1068098 BTC on 12W7RgjugzpULqwfq1tCzdU89LZnvVvJVw in transaction 1a9312d4e241fdede59aa4f436957f202c1672d3ebf6548d26d6510a31940b85
You created transaction 5b8ac337977fb158ae33e65640be6ad93dcc374d335f0c3c48980d8e866b35be with the previous input and created an output to 1HsZznSdPsthijT6NGnC6n5YKj7icg5CpY and to your change address 18rnXzMAzrD5wiR4Go91TPNJWVhhnnxnrh
Now, you've backupped your private key, but did you backup your wallet.dat??? I can only guess here, but if you really exported the private key for address 12W7RgjugzpULqwfq1tCzdU89LZnvVvJVw, but didn't backup the private key for address 18rnXzMAzrD5wiR4Go91TPNJWVhhnnxnrh, nor backupped your wallet.dat, you no longer have the private key to controll address 18rnXzMAzrD5wiR4Go91TPNJWVhhnnxnrh i'm afraid...
In short, bitcoin core isn't a deterministic wallet. It generates a random private key/address pair each time you send change to yourself (in reality, iirc, it calculates about 100 pk/addresses in advance).
wallet.dat holds these private keys for you. You can export one of them, but if you don't export all of them, you might lose coins.
Best thing is to do a regular backup of your wallet.dat, that way you're sure all your private keys are backupped.
Back to you, OP, if you want your coins back, you might try file recovery software... Shut down your pc, unplug your harddrive and see if some of the previous files are still stored on your HD. If you're lucky, you can find the old wallet.dat, altough there is no guarantee.