There could be many reasons why you're struggling with the melody, but it usually comes down to overthinking it. I don't think your progression is really complex either, so it should be easy to write! Also, nothing wrong with confidence, man. It's only arrogance if you can't back it up
If it helps, here's the quality control I put every vocal melody I write through, as I write a tonne for my band:
1. Is it clashing with what's underneath? - For example, is there a lot going on rhythmically, musically underneath, counter melodies on other instruments? If so, you'll want a vocal melody that gives the ear a break from that, so it can be heard clearly and the listener can latch onto it easily without having to listen hard. (An analogy for this would be going out to dinner wearing a really funky patterned shirt - you probably wouldn't choose a striped tie and polkadot jeans with it, you'd likely simplify the rest so that it's not sensory overload) - So basically, if the instrumentals are busy, then the vocals usually work better simple & vice versa.
2. Is it easy to sing/Fun to sing? - Do you struggle to pronounce the syllables? or do they flow well together? This is always something to think about that a lot of people overlook - catchy hooks tend to be really easy to sing so that even the casual music fan could sing them, which gets you more ears & makes it more memorable. This is the make or break part for me.
3. If you were to sing it without the song to a friend that had heard the song, would they be able to tell it was from your song? - This is where the catchiness factor is really put to the test - Think of how any one of us could hum a verse or chorus line from an A7X song and be able to know exactly which song it came from. Their verse/chorus melodies are clear cut & they repeat often, so that there's a clear verse melody & chorus melody - this is definitely something to strive for too.
My advice would be to leave the song playing and hum random lines/words over it until something natural comes out that you like, then record it and see if it works with you being the listener
Hope this helps!