Q. Do Catholics have to believe in the sinlessness and assumption of Mary?
A. Yes. We are obliged to accept and believe everything the Church teaches. If you are having trouble with any doctrine of the Catholic Church then you may have to simply choose to submit to the wisdom and authority of the Church. In other words, accept it purely on faith. But of course it is far better if you are convinced that what the Church teaches is true and makes more sense than any competing ideas. But this may take time for you to research the reasons that the Church teaches the doctrine you are having trouble with. You can ask a knowledgeable priest or research on the internet. And pray for God to help you find the answers to your questions.
I can guarantee that if you are honestly searching for the truth and not just trying to find fault with the Church, you will find answers that will be both simple and sublime. I have done this research many times and every time the answers are more intelligent, more logical and better documented than I had expected. I am thoroughly convinced by the evidence and thoughtfulness of Catholic theologians and don’t have to take blind leaps of faith.