I think you should see someone like a doctor or someone working with mental health first to make sure that it isn't a physical or mental issue that you're having. But always be aware, a lot of doctors and those types of workers will jump on the chance to put someone on medication, sadly. So just be careful with what you tell them.
But, we'll look into possession anyway to be safe.
1) It all depends on the type of spirit and the level of power the spirit has on its own and has over you.
Different spirits with different power levels can have a different effect on a person. Sometimes the spirit can take complete control of the person and the person will ether black out and not remember a thing, or they'll see themselves doing it or be aware of whats going on but now that their actions are not their own.
The amount of power it has over you also effects that, given the time the spirit has been in your body, the healthiness of your body, mind, and spiritual soul will ether weaken or strengthen the spirits power over you. If you're a person who's sick physically, has emotional issues or mental disabilities, or isn't spiritually protected or very strong soul-wise - the spirit will probably have better control over you. If you're a healthy person, with a strong mind and soul, ten to one, it won't be able to control you as easy.
2) I would say if its a harmful or dangerous spirit to yourself or any others, you should get rid of it. And even if it isn't, most spirits follow a code not to take over a living person's body without the person saying its all right for them to do. The body is yours, and you shouldn't let anyone have control over it. When you do, you're allowing them to do whatever they want with it. They could hurt you or another person (usually a family member, lover or friend), and/or they could do something to get you put away for a very long time.