About one day ago I sent a transaction with an average gas limit, but a set a price of 15 Gwei. The transaction is still pending nearly 24 hours later and is still pending; will it ever complete?
As the question is "will it ever" the answer is most likely "yes". 15 GWei is not a bad fee in general, we just currently happen to be in a situation where that is not acceptable. Sometimes transactions with even 0 gas price are mined (although the reasons for that remain a bit unclear).
So, as you were already told, just wait or replace the transaction.