Wait for an overcast day or block the actors away from the window.

If that's not an option you can use a black bed sheet or a black cloth.Don't use other color, it will probably mess up the color. use some sort of hanger or rope to block the sunlight while keeping the black cloth away from the camera's sight.

Be creative and good luck.

Edit: If black cloth blocks too much light then use some white surface to reflect some light back to the room.

Thanks for the feedback .Much appreciated .

Thanks for the review. I dont know if you noticed that the masked man and the boy are the same person (which is pretty obvious if watched closely) .
It's meant to be like a dream sequence.I guess I failed to clarify that its a dream sequence and got my audience confused and asking the wrong questions .
Any way thanks for the review again.

I'm assuming you want to be a Director.

At first you should read Robert McKee's "story substance structure style and the principles of screenwriting"
to better understand how story works. Then read "screenplay" by Syd Field to dive deeper into screenwriting.

Only after you understand how a story works , you should start learning cinematography (to communicate with the DoP) , acting (since you need to direct the actors) and so on.

It would be much easier to answer if I knew what is your boss telling you after rejecting your work. (ex. cuts are too short/long , pacing/rhythm is not correct )

You can of course watch other videos similar to your work and analyze them . (how long are the cuts , rhythm etc )

You said you dont have problem editing small vids like trailer, so try to treat each scene / portion like you would treat a trailer then look at the whole. Make necessary tweaks. It's just a thought , but it might help.

Good luck on your next edit.

Thanks for the advice and good luck on your first feature. :)