This will happen sometimes, and there's not much you can do about it. Much the same thing happens with cash. Say you have a $100 bill, and you want to pay me $10. In our transaction, the $100 will be split -- you'll get $90 back and I'll get $10. But while that's unfinished, you can't spend your $90 change from the transaction with me because it's not fully yours yet.
The exact mechanics will depend on what unspent outputs you have. If you only have a single output, then you will have no funds available until the transaction confirms. If you have multiple outputs and not all of them are needed for the transaction, then some of them will still be available before the transaction confirms.
Cash behaves the same way. If you have a $10 bill, a $50 bill, and a $100 bill, you can send someone $60 and still have the $100 bill available. Or you can send someone $65 out of the $100 bill and still have the $10 and the $50 available while that transaction is finalizing.