建议用 an indispensable part of (my/your/his/one's) life.
例句：
Mobile phones have become an indispensable part of our lives.
此外，可以考虑：
a vital part of life
a necessary part of life
以及网友提示的 an essential part of life
an essential part of a person's life
an indispensible part in one's life
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