一生挚爱懵懵懂懂的MM
[yī shēng zhì ài mĕng mĕng dŏng dŏng de mm]
The 'MM' here stands for 'Miss', a term sometimes used for young, pretty women online. The user is calling themselves 'a young lady loved for life by someone', with '懵懵懂懂' describing the person as slightly naive yet lovely.