My opinion is that you are struggling with too many things at the same time. Going for anime or manga right from the start will slow you down. I'd let go the manga style for a while and focus in the basics of drawing such as proportions and anatomy.
Then you can go and transform the body as you please and it will look much better. Make it manga or whatever style that uses different proportions.
And about using sai, its the same thing. For starters drawing in pencil is different from drawing in a tablet since a lot of your skill comes from your muscular memory. Your skill wont be transfered from one media to the other just like that, it will take you time to get used to it. So focus in only one, and i'd recommend pencil since digital programs are way to much to handle if you want to focus on basics.
Otherwise, its not bad at all, but i recognize some things that are common in the people that jump right into manga like drawing one eye and covering the other with hair because you cant just get it right.
In any case, just practice, use photos for references and dont stop drawing.