Is it dumb to jump into another serious relationship right after getting out of one?

I think it's imprudent. Wait for a season, however long that season might be. If you've got someone who wants to jump into a relationship with you and she's not willing to wait it out with you, let her go.

Rebound

Maybe. It depends on how serious of a relationship or what you mean by "serious" relationship. It also depends on how you really feel because it can be a big step in life.

Yes it is it takes time to build a relationship

expectations are rarely ever met.

If you are insecure, then your own sense of self worth is so bottom, that you will do this.
Fear of been alone, drive people into another one.

.………………..YES.

It might be dumb but I did

Very dumb.

Dumb as fuck