The line "I walk upon the thorns of life" follows an iambic meter, as it has a pattern of alternating unstressed and stressed syllables. The stress pattern goes like this:

• I walk (unstressed, stressed)
• upon (unstressed, stressed)
• the thorns (unstressed, stressed)
• of life (unstressed, stressed)

Each pair of syllables (foot) starts with an unstressed syllable followed by a stressed one, which characterizes iambic meter.