First of all I think you need to be less harsh on yourself, While each thing you picked out might be true on its own, the overall image Is really nice. the character is cohesive and nothing immediately jumps out as wrong. and because it's an alien/monster you have more wiggle room. but I can give some feedback, rather than the forehead being too big the hairline is too far up, bring the hair line down and let the curls hang lower and it will look great. you said the eyes are too big, I think they could be a little wide, but ultimately this is dependant on the style your going for. look up some artists who use a similar style and copy their eyes until you know what you like. you say she has curlish hair, is it supposed to be more or less curly? If it's supposed to be more curly than, I would say its wavy, and to fix it look up references of curly hair. or add some loops/spirals and volume curly hair tends to have more volume. Unless your going for a 3 foot adult, most people when drawn alone have a fairly ambiguous height. plus you said she was an adult so the proportions match that of an adult. if you want to experiment with height then add more people to the scene. she doesn't look like a child, she could be anywhere from 16-26, the lines on the forehead make her face look a little older. no problem here you say ears are droopy and long like that's a problem, I personally like them, they fit the expression of the character, and create a sombre/melancholy feel. only change that if you want a different expression. the neck is fine, you could make it narrower a tiny bit where it touches the head, but again this is a style problem, the more comic/anime style of the character makes it work. again look up art you want it too match and copy from them. the depth problem is also a style thing, but you could also play around with lighting/shadows to add depth, find a 3D model with light and shadow that you can reference and try adding those to the face. the reason it looks flat is because of that and the front on pose, other angles will more naturally lead to a deeper look with the nose bridge and curve of the skull. once again its a great character drawing so the best piece of advice I can give is draw more of them, and have fun doing it.