18 year old, been mewing for about 5 months, I think I can help you. 

One of the largest and most difficult hurdles to overcome when starting things like this, especially mewing, is uncertainty. It is difficult to tell whether you are doing anything right, and it can certainly be discouraging at first. 

1.) Do you have a social media or somewhere that I can send you a video? If you have Instagram, message me @liiam.grey and I will show you how to tell. It is difficult at first, and you will likely be unable to breath. I will help you with this as well.

2) Yes, your the bones in your skull never fuse together, not even in adulthood. You will notice minor improvements almost immediately, as oral posture is so so so important for so many things, but the big changes that you strive for will take months and years even. However, within that time, you will notice changes, and there will be constant improvements. So please, never be discouraged. I can send you a video of a man, 19 who completely changed his jaw and bone structure in only 8 months.

3) There are plenty of excersises you can do, not only for mewing but for general facial health. I will compile a list, and post it here. 

There isnt much I can do to answer your questions without showing you, so please get in contact with me because I would like to help you.

can you also show me the videos ? I dont have instagram unfortunately

send the video to my  @mr.mewing