Why can't I be re-baptized as a Catholic?
The short answer
Baptism is given once for all: it washes away original sin, gives sanctifying grace, and marks the soul as Christian. Repeating a valid baptism would be useless and an objective sacrilege. If you were validly baptized, you are a Christian; confession is the sacrament for sins since then.
Once for all
Baptism washes away original sin, gives sanctifying grace and imprints a supernatural character that makes a person a Christian. It cannot be repeated.
Why not redo it
A second baptism would not "take," because the first was valid. It would also be an objective sacrilege, casting doubt on the first.
You are a Christian
Even if you have not lived as a Christian until recently, if you were validly baptized you are a Christian; that doesn't depend on how well you have lived. The sacrament for sins committed since baptism is confession.