Why I always fall in love so fast and when I got to know the person I lose interest?

OK.My situation is this: I'm having a hard time having a relationship.The problem is that I'm too picky.And,too impulsive at the same time.I can chase somebody very long and when he starts paying attention to me and I see his real personality,I began to lose interest.Is it because I'm not ready for a relationship?I want to have a relationship,but it's hard for me to find the right person.And everytime I think I did, either I find somethink annoying in his character,or he just wants me only for . I also am ve ry expressive person:when I feel good vibes about someone I tell him.And then I think I love him,but in awhile I lose interest when he tells me he loves me too.And,when I'm in relationship and I know we love eachother,I always keep my self open for another,better, gyu.Just incase if we brake up.I want to stop this,but I don't know how?Any advice?