Nice portrate Cristina, you've caught the persons character very nicely. You just need to get the heads proportions correct, and there will be no stopping you.

Eyes are always on the halfway line between the top of the head and the chin. The base of the nose is always at the half way point between the top of the eyes and chin, and he mouth is normally halfway between the halfway point and the cin, unless they have a large jaw or chin.

Another thing to remember is the ears, they attach to the head level with the eyes, but seem to be higher if the head is looking down, or lower if it is looking up. The base of the ears can be any where between the line of the base of the nose or mouth, just depending on their size.

Hope that helps, try using your thumb to size them up preoperly, I know it looks affected but it isn't. Artis don't do anything without reason when it comes to painting.. Hugs