Honest questions about Catholic faith please?

Hi! I have been so excited about converting to Catholicism. I know for sure God has called me. I am supposed to start RCIA cles in September & be confirmed next Easter. The questions I have are in all sincerity. I have been studying, and I hesitate to ask all of my questions, but I do want to know a couple of things. PLEASE do correct me if I am wrong! Is it true that Catholics have 2 types of sin & one is considered unforgivable...grave sin? This includes not being present at M once a week? And, if one does skip any weeks, they are considered as falling away from grace...which means automatically going to hell? I believe if we accept Christ as our Saviour we are going to Heaven by grace...However, I do agree with most everything I have learned about the Catholic faith & I feel very drawn to it but I am stumped by the thought of being doomed by the church if I am a member? I realize the other faiths were manmade doctrine that originated out of Catholic Priests who no longer agreed with the Catholics. Luther, Wesley, etc...What else causes a Catholic to be unforgiven? Or, am I misunderstanding this?