I did a comprehensive ultrasound at week 19 of my pregnancy & she told me I'm having a girl for sure & could it change later? I really want a girl

Nothing's for sure. At 19 weeks, the little male parts are developed enough to be seen on ultrasound. If the sonographer said "for sure" then you can bet she saw the cute little area and noted the absence of a penis. Will one grow out, later in pregnancy? Absolutely not. Could the technician have made a mistake? It's possible, though not likely. So I'd go ahead with your girl colors and decorations! Congratulations!