It is so awesome to hear a question like this. My spiritual adviser said that a person who commits a sin of this nature and who is concerned with if he offended God in a mortal way, is on his way. As Jesus said, “you are not far from the Kingdom of God Joseph of Arimithea”. My adviser also said, "it is the soul who uses the name of God in vain regularly with no concern that we must pray for, because that soul is lost, and needs help finding his way back.
We must all keep in mind that the three conditions for mortal sin are not grave matter which is “accepted as wrong” and then done deliberately against God. It is grave matter, which you have been told is grave (whether you agree or not) and you continue to do it anyway. So if the church teaches birth control is a grave sin and you do not want to accept this teaching because it impedes on your way of life, the sin is still grave and you now have full knowledge. If you now do it freely anyway, it is Mortal.