The main reason is probaby because I'm french and I write in english. But even when I tried t write in french, it was hard ... and less melodic !
I tried to find some ressources to improve my lyric-writing abilities and I found out several things, but one of the best is an audio course by Shamir Rele, called "Songwriting course". It's an interview he made with a professionnal songwriter, who explains very deeply the various topics related to songwriting.
I found this to be very helpful to me, and this gave me new ideas and perspectives about my songwritings.