It's possible/likely that some of your inputs have already been spent.  You might recall a prior error you had when sending an amount beforehand.
Try sending smaller/different amounts to get out as much as you can.

Several other reasons this could have happened as well.  There isn't a simply 'press-button-fix' for the various reasons.

All that said, you can send me a backup of the wallet, and I can try to recover GBB for you.  I've done this for others already.
In worse case, you can't recover anymore, but I can assure you that you will not get everything the wallet shows, since it's a misrepresentation of what you actually have left.
Also, since you're already moving your funds to another wallet, there is no fear of giving me your wallet.
(Of course there has to be 'some' bytes to pay transfer fees of the blackbytes though.)

If you sent, but the other wallet didn't receive, you can re-send the private package as well (although this doesn't fix your wallet).

PM me if you want to try.