I joined the Catholic church almost three years ago (Easter 2010). I will be 53 years old in February. I was raised Methodist and was baptized around the age of 8 to 10 years of age by sprinkling. I later lived a very sinful life til around age 23 or so, when I was re-baptized by immersion as a member of the First Church of God. I was raised to ask forgiveness of my sins directly to God through/ in Jesus name. I married a lifelong Catholic about 2 years after my second baptism. We took the premarital classes and married in the Catholic Church. I attended mass every week for over twenty some years with her. We raised our two sons in the Catholic Church, sent them to catholic schools from k-12. I made the decision to convert on New Years Day 2010 and came into the church at Easter. Most everyone thought I was already Catholic due to my attending mass all those years. The priest that married my wife and I was also the same priest that I made my first of two confessions with and I came into the church with. I tried my best to confess the sins I could remember from the time of my second baptism. Now I have a question. Were my sins forgiven after my second baptism? Our creed states 'one baptism for the foregiveness of sins'. Since this was my second (and last), were my sins forgiven? I took this baptism seriously, which was by total immersion in the name of the Father, Son, and the Holy Spirit. The first sprinkling in the methodist church I didn't take too seriously; although I should have. Again, it made little difference in my life as I went on to a very sinful youth and young adult life until my second baptism. I have changed my life thanks to the grace of God, and I'm nothing like I was before. I have lived the best Christian life I can for the last 26 years or so. Were my sins forgiven when I was baptized the second time? (upto that point in my life)
The Catholic Church does recognize Methodist baptisms as valid. As you noted, the Creed clearly states that there is only one baptism. Your baptism in the Methodist community was your one baptism. Your second "baptism" was, in the eyes of the Catholic Church, not sacramentally effective. You should confess all sins committed after your Methodist baptism.