Well even if it feels like a lie, write about yourself positively anyway. It's work, not a relationship, so don't even feel bad about it. You don't have to believe it yourself to market yourself. Treat it like something depersonalised - what would a marketing person write to sell you? Take a look at your work records, documents, emails - anything that got praise or stood out? If you don't believe yourself, take what others have told you as your starting point.
I think it would help looking at who's voice it is that you hear. Who in your life made you feel this way about yourself? Because you sound awfully harsh with yourself. Is that voice realistic or is that voice also bullshitting you? What's it's goal? To stop you from feeling good about yourself? To save you from disappointment if your own view of yourself doesn't align with what other people see? To keep you small and compliant? Dig into it a bit, maybe that helps.