in her own blood. These all seem to be in a realistic vein. He seems to reject reality in the last sestet when he says that he forbids Time the crime of aging his lover. This is a cry against, but also an accurate rendition of what happens to us: Time writes his lines on us, we age, and we die. Most of the imagery accurately represents reality. The last two lines are symbolist in the artistic sense, because his subjective reality predominates: she may be destroyed on the outside, but she will remain immortal because he writes of her.

The “long-liv’d phoenix” symbolize our desire to escape the ravages of time. Associated with Egyptian sun-worship, it had a life span of more than 500 years. When its first life was over, the bird would burn itself upon a pile of wood that was set ablaze by the sun, and then it would then rise from the ashes, once again young. Here Shakespeare is saying that, despite the phoenix's ability to resurrect itself, it cannot escape Time forever. Metaphors dominate the poem, and if you want to discriminate between symbol and metaphor, your discussion is more limited.  Metaphors include the personification of time, the paws of the lion (the lines time makes on the face), the personification of earth, and so on.  The speaker does not want his lover to show signs of aging, but argues that even if time does its “worst,” his (the poet’s) pen (poems) will continue to preserve his love/youth (many consider this poem written to a male lover). In so far as Shakespeare uses images from nature, they would best be described as naturalistic.