Keep your promise and never change. Thank you! !

Since it is an advertising language, it should be concise.

I think the most appropriate translation of "promise" is promise.

My advice to you is: let's stand by and watch our promises, forever!

Supplement to the question:

I probably know what you mean, so I can't mention' we'!

Two other suggestions, rhyming:

Stick to your promise and never change!

As always, never waver!

Adhere to the eternal promise!

English niurentuan