Credit card issuers sometimes give a lower interest rate to some of their best customers, but you have to ask for it. Here's a sample script you can use to ask for a lower interest rate on your credit card.
Customer Service: How may I help you?

You: I’ve been a good customer with you for several years, but I really need a lower rate on my credit card. Can you help me?

Possible Customer Service Response: Let me see what I can do for you.

I’m allowed to lower your rate from 15% to 9%. Will that work for you?

You: Yes, that’s great! Thank you.

Another Possible Customer Service Response: I’m sorry I’m unable to lower your interest rate at this time.

You: That’s fine. May I speak with a supervisor?

Then repeat the script with a supervisor. If the supervisor can’t or won’t lower your interest rate, then simply say thanks for your help and wait a few months before trying again.