I did not get the full 12 months free period

There are several reasons why you might be getting billed despite thinking you're within the AWS Free Tier period:

1. Exceeding Free Tier limits: The AWS Free Tier provides specific usage limits for eligible services. Even during the 12-month period, if you exceed these limits (like using more than 750 hours of t2.micro/t3.micro EC2 instances per month), you'll be charged for the excess usage.

2. Using non-Free Tier services: Not all AWS services are included in the Free Tier. If you're using services outside the Free Tier offerings, you'll be billed for them regardless of your account age.

3. Using resources that continue to generate costs: Some resources incur charges even when not actively used. For example, stopped (but not terminated) EC2 instances may still generate charges for associated EBS volumes or Elastic IP addresses.

To address your high costs during development:

1. Check your AWS Billing console to identify exactly which services are generating charges.

2. Ensure you're using Free Tier eligible resources (like t2.micro instances instead of larger ones).

3. Properly terminate (not just stop) resources you're not using, including deleting any unused EBS volumes, snapshots, or releasing Elastic IP addresses.

4. Consider setting up AWS Budgets to alert you when costs exceed certain thresholds.

5. If you believe there's been a mistake with your billing, you can contact AWS Support for clarification.

Unfortunately, AWS doesn't typically extend the Free Tier period beyond 12 months, even for development purposes. However, you might want to explore AWS's "Always Free" tier services that remain free indefinitely, or look into services with short-term trials that might still be available to you.
Sources
Free Tier Extension Request | AWS re:Post
Confused Billing | AWS re:Post
Billing before the end of the free period. | AWS re:Post