Please help me, I feel like I'm going insane..

Sometimes I feel extremely creative and happy, have all these ideas/thoughts etc. But it feels like I don't know how to express them.. I want to do them all at the same time, it drives me crazy!
Whenever I feel like that, I also feel extraordinarily great. (like I have all the powers in the world, please don't take this the wrong way) I make poor decisions, like re-decorating my whole house in the middle of the night.. or spending too much money on things I don't need.
But I can get so irritated and hyper at the same time.. and there's this side of me, that scares me. Like making a hit list, thoughts of hurting people who once did me wrong. These thoughts are most of the time violent. I think that no one will catch me, that nothing will happen to me. Because I feel so powerful. And that there's nothing in the world what I CAN'T do.
But, all that I just described will fade, and then I will feel hopeless, worthless, hurt and deep deep sorrow. Have suicidal thoughts. Self-harm and cry all day, hating my reflection in the mirror. Regretting all my decisions, feelings so ashamed for who I am, I don't feel like waking up, cleaning, shopping, eating and even taking care of myself.. This goes on to the point where I just feel numb, I can't cry or feel anymore.

Please help me, what is wrong with me. This just can't go on like this, I know that. I don't want to hurt someone nor myself.. I know all of you guys will say that I need to see a therapist, but I don't know if I'm overreacting, or that I am exaggerating. A therapist is meant for people that really need the help, not for someone who is just overreacting.

So am I overreacting?? Does anyone know what could be wrong with me??
Please help, what should I do?