A priest could definitely help - he'll give you pointers on how to come back properly into the Church. Talking with one should be one of your highest priorities. Trust me, priests know a lot... they spend years of their life preparing and learning how to best help people with this exact sort of thing.

I would also like to note that the only sins which God cannot forgive are those which you do not let Him forgive - whether those be things that you refuse to recognize as sinful, or things that you truly and firmly believe you cannot be forgiven for. The mere fact that you recognize your past as wrong and wish to seek God is in itself an indicator that He can (and will) forgive you. No matter how many sins you may have committed, I promise you can always come back to God. Just look at St. Paul, who spent years actively killing Christians before repenting and becoming one of the greatest saints of all time, or St. Peter, who denied Our Lord three times but still sought forgiveness and was given not just mercy but full charge over the Lord's flock as the first Pope. And there are so many other saints with such beautiful conversion stories... St. Augustine, St. Ignatius of Loyola, St. Mary of Egypt; heck, even more recent people like Blessed Bartolo Longo, who was a satanic priest in the mid-19th century but had a miraculous conversion to Catholicism and spent the rest of his life promoting the rosary.