Hi sung,
At 10 years old I think your Serta has probably reached the end of its useful life. A topper can help if a mattress is too firm and you need more pressure relief but it it not an effective way to improve the support of a mattress which has softened except perhaps partially or temporarily. You can read more about this in post #4 here
I would personally look for a new mattress rather than trying to “rescue” a mattress that has lost its support.
Phoenix