Martin, it’s not a recommended practice to change passwords monthly. Many companies impose stupid policies like this in the belief that it’ll increase security: it won’t. You’ll get users who, out of frustration, choose a password like: MartinJAN1 or MartinFEB2. This reduces security!
Do what Svante says; remember a long complicated password and don’t change it unless you have reason to believe it has been compromised.
If you have a bad memory then use a password manager. Having a unique password for every website you visit is a highly recommended practice as even the best of us can’t remember a 25 alphanumeric password with symbols in there too.
Have a read of this: