これからもずっと、一生の宝物として大切にしますって英語でなんて言うの？

"Thank you so much for the gift! I will prize it!"

You begin by expressing thanks for the gift you have been given.
Then you could tell them that you prize it. "Prize" a gift, something highly valued, hold in high regard, cherish, etc.

"This is a special gift, I will always treasure it!"

You acknowledge that the gift is very special.
A treasure, something you hold dearly, greatly value.

A treasured possession is something that is very valuable to you.
It is not necessarily something that is worth a lot of money.
Many times it is valuable to you for personal or emotional reasons, and it is often something you got from a person who means a lot to you.

>Thank you for the nice gift, I will value this forever.
value=the regard that something is held to deserve
..............................
>Thank you for the gifts. I will cherish it forever.
cherish=hold (something) dear.
...........*...............*..............*............

example
"thank you, it's beautiful. I will always treasure it".
or
"i will treasure it".
or
"it's a valued and treasured possession"
to treasure something is to love it and give it a
status of a special or valued gift.
"i will always treasure it".

Treasure' can be described as valuable goods. Whilst this usually refers to traditionally monetarily valuable goods, such as gold, diamonds and other precious gems, it can also be used to describe items that are hold personal value to you. For example, a painting a child has drawn for their mother, could be considered a treasure to the parent.

We use the term 'forever' when we mean a very long time, or until the end of your life. It does not literally mean until the 'end of time'.

Please note that the word 'treasure' can be used both as a noun and as a verb. In the first statement, 'treasure' has been used as a verb: I will always treasure it. However, in the second statement, it has
been used as a noun: I will store it as one of my treasures.

When you decide to keep something as a 'souvenir', you may not use it for anything else. You will keep that thing as a reminder of a person or a place you visited. In this case, if you keep the gift as a 'souvenir' it will always remind you of the person who gave it to you.
So, you may say:
Thank you for the gift. I will always treasure it for the rest of my life.
or
Thank you very much for the gift. I will store it as one of my treasures for the rest of my life.
or
Thank you very much for the gift. I will treat it as a souvenir for my whole life.