A more universal solution is the "child pays for parent" method. The good thing about it is that you don't have to wait until your initial transaction is "forgotten". You can use it right away. But as @jrickord mentioned it is not really obvious how to apply it.

As summed up on stackexchange by the user maservant: "Child pays for parent means, as the name implies, that spending an unconfirmed transaction will cause miners to consider confirming the parent transaction in order to get the fees from the child transaction included in the same block."
For a detailed step-by-step how to with Electrum, check this guide

In short:

• First, find out if an output of the unconfirmed transaction went to yourself by checking the details in the "History" tab. Usually, this will be the case because one of your addresses receives "change" from the transaction. Write down the hash
• Then, go to the "Coins" tab, select the corresponding hash, right-click and select "Spend from Address". This will take you to the "Spend" tab
• Here, enter one of your own receiving addresses. Thus, you will create a new transaction, containing as an input part of the unconfirmed transaction. When miners confirm this new transaction, the former, unconfirmed one has to be confirmed as well. This is what solves your issue
• Add a fee high enough to make both transactions worthwhile for miners. To do so, check https://bitcoinfees.21.co/ and apply their recommendation to the combined size of both your transactions
• If your "change" address does not contain enough coins for this, you can transfer the total amount of your respective wallet instead. This will also include your unconfirmed output (if there was an output to yourself) as an input to the new transaction
• Broadcast the transaction. When it is confirmed, the former transaction should be confirmed as well