Can someone help me translate to latin?

It depends on what the gender is of that which is "yours".
Semper tuus -- masculine
Semper tua -- feminine (probably the case here, based on your username)

I want to have it engraved on a watch for him. Don't understand what you mean with the gender of yours. Could someone explain this to me? Would hate to engrave the wrong thing.
Thanks

Sorry. Let me try to explain.

The word for "yours" in Latin has different endings depending on whether
it refers to something that is masculine, feminine, or neuter. Those are
the three genders.

Suppose I am saying "Forever Yours" to my Lovely Bride. I mean to say
to her "I am YOURS". So "YOURS" is masculine here
because it refers to me. In your case, of course, the reverse is true.
"YOURS" refers to you and you are feminine.

So the Latin phrase is

Semper tua

"Tua" is the feminine form.

Now let me say something that I say to anyone who wants to commit a
translation to some permanent form such as an engraving or a tattoo.
Please have someone else review it. I'm not a professional Latin scholar,
I just do this for fun.

Best wishes on your wedding!

(If you don't know who to ask to review it, you can try
The International Forum.
They have some very knowledgeable folks there.)