I don't necessarily think that it's "bad" to want to keep your past a secret from your new beau--sometimes it can be scary to expose dirty laundry, especially when you're just starting a new relationship and want to highlight your assets. In that sense, a little deception is natural, almost Darwinian, as it's often the female bird with the brightest colors who draws the male attention. That said, I have some concern about your approach.
I'm concerned that this is not merely about your desire to temporarily keep information from your boyfriend, but that you feel a need to permanently cordon off a part of your life about which you have a great deal of shame or discomfort. If that's the case, I would strongly advise against such a tactic. You can't build a strong house on a weak foundation, so the saying goes. When you block off a part of yourself and your life from your partner, you weaken the strength of your bond and create an unnecessary wall that will ultimately undermine the depth and strength of your connection.
It's ironic that people often keep secrets from their partners because they fear that disclosure would put their relationship at risk. But what often ends up happening is that the information they withhold becomes an entity unto itself that gets bigger and bigger the more it's fed by silence. It takes a great deal of energy to keep secrets; that wasted energy drains intimacy between a couple because it makes the secret holder unavailable for a more authentic and spontaneous connection. A relationship is bound to suffer under such strain and ultimately may fray or implode. If the relationship is maintained, it will likely be characterized by coldness and rigidity, an emotional hollowness.
So, if you truly want to give your relationship the chance to grow to its fullest potential, I encourage you to tell your secret some time in the near future and risk the consequences. If this new boyfriend is worthy of being something more to you, chances are he will be able to tolerate the truth, even if it might be unpleasant and uncomfortable.