That address shows as having received no coins on all the block explorers... is that address you were attempting to send coins to from Bitcoin Core? Or is that your address in Bitcoin Core that supposedly contains coins? 
If you right click on the transaction in Bitcoin Core, and select "Show Transaction Details"... what does the "Status" line say?
From the 18th of March to the date Bitcoin-core Wallet app retains my satoshis with the
message "this payment has not yet been transmitted";
This message sounds like you've attempted to create a transaction, but it was refused by the network... so it technically hasn't been broadcast yet and it doesn't exist on any nodes.message "this payment has not yet been transmitted";
If you right click on the transaction in Bitcoin Core, and select "Show Transaction Details"... what does the "Status" line say?