I got a new phone and I forgot my Apple ID to my last phone and now I can’t log in to my apple watch

You can try this steps If you forgot your Apple ID password - Apple Support

Otherwise, if you have proof of purchase information you can start a request at Apple Support: How to remove Activation Lock - Apple Support

You can ask apple to remove the activation lock online on this page;

Remove the Activation Lock Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.