Start with the statement
Every subscription ends up in the same place: your bank or card statement. Read two or three months of it and note every charge that repeats, including small ones and ones with names you don't recognise. The name on the statement is the company that bills you, which is not always the app you remember.
If a charge says Apple, Google or PayPal, the real merchant is one level down, in that account's subscription list.
Subscriptions billed by Apple
On an iPhone or iPad, open Settings, tap your name, then tap Subscriptions. On a Mac, open the App Store, click your name at the bottom of the sidebar, click your name again, then Subscriptions. Apple's support page also covers Windows and the web.
If something is missing, Apple suggests searching your email for "receipt from Apple" or "invoice from Apple" and checking which Apple Account the receipt names. A family member's subscription can only be cancelled by them.
Subscriptions billed by Google Play
On Android, go to Subscriptions in Google Play while signed in to the Google Account that pays. Google notes that uninstalling an app does not cancel its subscription, which is one of the most common ways a charge outlives the app.
If you can't find a subscription there, Google's tip is that it may be on a different account, so switch accounts and look again.
Automatic payments through PayPal
On the PayPal website, go to Settings, then Payments, then Subscriptions and saved businesses (or Automatic Payments, depending on your account). Each merchant listed there can charge you without a new checkout, and the page lets you cancel the agreement.
Anything not billed through a store or PayPal is billed by the company directly, so the statement is your only map. Cancel with the company that bills you.
The one you might keep
Forget Your Credit Card is not a cancellation tool. It is the other kind of subscription: one that admits up front it does nothing. You pick a monthly amount from $1 to $1,000, Stripe bills it, and confirmed payments count toward a public Lifetime board if you choose to appear.
It shows on your statement like anything else, and you can cancel it in one click. The difference is that nobody forgot to cancel it. Forgetting was the point.