Anabaptists and OSAS

 Having come from an anabaptist background, we believed in the necessity of baptism for salvation. But we also believed that there was only one baptism that we needed to partake in. One question that I had which made Catholicism more plausible to me was: how are your sins forgiven post baptism? Since baptism forgives sins, and you can only do it once, how is it that sins that you commit after your baptism are forgiven? You could confess to God, but surely you could do that before baptism as well. This made me believe the sacrament of confession made more sense. But if I were to remain a protestant anabaptist, how could I answer this? 

One answer could be that baptism works preemptively to forgive future sins. But this leads to a doctrine similar, if not outright the same, to the Once Saved Always Saved. That is to say, once God has forgiven you and written your name in the book of life, you are there forever. But I suspect many anabaptist would recoil at this solution. What other solution is out there, though?