sort of confused but i'll try to help you
1. we are in the physical right now, when you project, as in first leave your body, you're in the ethral [spelling?] i think of it like the spiritual plane since it's still in the physical, but you're more like a ghost. there are of course limits to what you can and can't do in the ethral, but it's good to float around, try to explore your house, to try and get a feel of things when you first project. when you're ready to go to the astral plane, just think of it. myself i picture a field with a path leading to a cave and mountains in the distance. of course you can picture something else, and go wherever else, or just think 'astral' it will send you there pretty much instantly. when you wish to return, think of your room, then your body. thinking of your body gives you a bit of a 'crash landing' and it's a pretty bad shock to your system.
2. i think it comes down to your physical self. if you had a heavy meal, it can effect your projection, how tired you are, some people don't project with a blanket over them as they feel it weighs them down. another reason could be subconscious, you're worried something might happen, or there's something on your mind preventing you from projecting. keep a journal, before projecting write down something like '8:00, last ate at 6:45, had a chicken caesar salad, rooms cold.' then when you're done, write down your results, it might help you figure out why sometimes it's harder for you.
3. while i've heard of astral possession, haven't heard of any cases because [and what's probably happening] you have a shield up to protect your body, anything comes near it that poses a threat and you're instantly pulled back from wherever you are. there could be something, it could be you're afraid of something, or something on your mind, and so on. like with the above reasons, you could be unable to project for many different reasons. try cleansing, and keeping a journal to record things.