Why can't I spend my bitcoins when they have been fully confirmed?
I don't use MultiBit, but just checked its source code to see what the message you quoted means.

It turns out that the message is shown when the amount you're asking to send plus fees is less than the "available balance", which is defined as:


  Balance that can be safely used to create new spends. This is all confirmed unspent outputs minus the ones spent by pending transactions, but not including the outputs of those pending transactions.


The source code also says:


  Consider a real-world example: you buy a snack costing $5 but you only have a $10 bill. At the start you have $10 viewed from every possible angle. After you order the snack you hand over your $10 bill. From the perspective of your wallet you have zero dollars (AVAILABLE). But you know in a few seconds the shopkeeper will give you back $5 change so most people in practice would say they have $5 (ESTIMATED).


So my guess would be that your balance includes some unconfirmed change from your previous transaction(s), and that simply waiting for that change to confirm will solve your problem.

In other words MultiBit doesn't allow you to spend the $5 change until you have it back from the shopkeeper.  Note that the Satoshi client does allow you to spend unconfirmed change outputs.