A transaction has not failed even if it does not confirm. You should never make a new transaction, but replay the first one until it confirms. TheThe only time a transaction will not confirm, is if the signature is invalid (not possible to do with wallet), or if the address (or one of them) you spend withfrom have a lower balance than the transaction requires.
Every time you sign a transaction, you expose parts of the private key for all the addresses used in the transaction. EvenEven if the transaction stays pending, the key is exposed, and a new TX will make it wors by exposing even more of the keys.
And to the question about sending again. The features of the IOTA wallet changes a bit every time, but you cannot use money you don't have, so if you have 6 Gi in first address, and first tx is to send 3 of them somewhere, all 6 Gi is locked in that tx, and the remainder will not be available before first tx is confirmed anyway. And And if the remainder is 'on its way' to an address, you should not sign any other tx out from that address wither, as this would cause you to receive IOTA to an address you have signed from when the remainder arrives.
In short - IOTA-wallet should not be used to make more than one transaction at the time. Make it confirm by reattaching/changing nodes etc before attempting to make a new transaction.