Lets address the hair length first.
If you hair is getting shorter without been cut then it is obviously breaking away, which will make hair shorter. Have you been using straightening irons a lot? ...More >
It is important to visit a professional salon every 6-8 weeks to maintain your hair condition and they will be able to advise you of which products to use to maintain this while at home.
The parting at the back is a strange one? If it is one length then I would introduce some layers in a technique called internal diffusion. This creates soft internal layers without any visible cutting lines. This will definitely help sort out your problem at the back of your head as well as making your hair much easier to style.