I am married to an American Muslim and we live in America. He does not provide for anything but food. He treats me rudely, threatens with divorce all the time and reproaches me because I am barren. Now, I feel very bad and regularly see a psychologist. I miss some prayers because of taking medications that cause drowsiness. Also, I want to take off my hijab. Is removing my hijab a major sin?

First
Marital life

You must know that God Almighty never burdens a soul greater that it can bear and He did not bring us to this world to be miserable. Therefore, you must think well of God most High.
· It is permissible for you in the Shari'ah to file for a divorce if you cannot bear going on your marital life with that man since you are not obliged to live a miserable life. This man may be good, however the Sahri'ah does not oblige you to continue with him especially when the matter extended to psychological disorder and neglecting the teachings of Islam.
· You must be strong and abandon this case of frustration and open your heart for life and be persistent in searching for sources of happiness.
· Perform the prayer of guidance [Istikhara] and ask God to guide you to that which is best, i.e. either to continue your marital life with that man, or to ask for a divorce in order to start a new happy life.

Second
Prayers

There is no harm in missing them when you are sleepy. You must make up for the missed prayers as soon as you wake up.

Third
Hijab

Hijab is obligatory for a Muslim woman. You can put it in a way that covers your hair and chest.

To conclude:
You must abandon this case of frustration that may destroy your life. Put your trust in God and think well of Him since He is the Ever-Generous Lord Who brings us nothing but happiness in this world and in the Hereafter.