Insecure and don’t know how to change it

I’m incredibly insecure about how I look and my relationship. My s/o is incredibly supportive and tells me all the time how much he loves me and how beautiful he thinks I am but for some reason I can’t get out of my own head. The saying “you accept the love you think you deserve” resonates with me a lot because I don’t feel like I deserve it and I really don’t know what changed in my life to make me feel this way. Any and all advice is welcome. I just don’t know how to start to love myself again.

(1st pic (01/2017) 2nd (04/2018) 3rd (12/2018))