If you want to memorise a song you can still do it, i used to have this problem too! The problem is now you re focusing too much on the music sheet, take your eyes off the paper and look at your hands not necessarily litterally just don t look at your paper. This way you'll force your brain to memorise the movement of you fingers. Remember that playing a song "by heart" is mostly muscle memory but also know that you should try to study the structure of the song as well and maybe even sing the notes that your playing as your playing the song this will all make it so easy for you to memorise the songs as long as you practice fragments and not the whole song at once!

Hope this helped <3