Looking for how to resolve pending transactions?
Pending transactions that are displaying as such on the blockchain (make sure to check a block explorer) can be replaced by sending another transaction with the same nonce, using the method here. This article should be your first destination if you're looking for how to replace a pending transaction.
One of the most common reasons your replacement pending transaction will fail is because you already have multiple pending transactions.
In these cases, you need to replace the oldest transaction first, and proceed through them in reverse order (from oldest to newest).
Age is determined by the transaction's nonce, a unique number (number used only once) that is assigned to every transaction sent from your wallet. Nonces are assigned in ascending order — so the smallest pending transaction nonce you can find is the oldest.
How do I find pending transactions?
If a transaction is pending, it will be marked as such in your MetaMask transaction history. You can also inspect your history on a block explorer.
If you try to replace a pending transaction before resolving older outstanding pending transactions first, your new transactions will continually fail. This is because you need to resolve the oldest first.