First, welcome home! I am quite happy to hear that you are coming into the Church. May God bless you and the Holy Ghost guide you through the process.
I am also glad to see you are trying to amend your ways. This is, perhaps, the most important aspect of getting sins forgiven. You see, you cannot just sin then go to confession. You must really want to change and not commit the sin anymore. That is what you are doing here.
While using the Lord’s Name in vain and other forms of blasphemy are always grave sins, the Church has recognized that there is some mitigation when a grave sin has become habitual. Just because something is habitual does not make it okay. What this does mean is that your culpability for the sin is diminished in some degree.
If you recall from your RCA, a mortal sin requires 3 things to be true. (1) Grave matter, (2) knowledge of the seriousness, and an (3) a conscious act of will after considering 1 and 2. (I know this is not exactly the language the Catechism uses. Please correct me if I mis-stated it.)
When something just “falls out of your mouth”, it is reasonable to think that number 3 above is not always met as you did not consciously consider it.
On the other hand, names are very powerful. There is no other name in Heaven and Earth that is higher, more sacred and more powerful than the Name of God. I have come to believe that this particular sin is so serious and so offensive to God that any mitigation is slight. Because of this I would say that using the Lord’s Name in vain is virtually always a mortal sin. (I will probably get hammered hard for this…)
I say this as a convert myself. I spoke, I suspect, much like you did, every 3rd word was, well, offensive to God (and probably many people around me.) My conversion was sudden and miraculous. One of the gifts of my conversion and a sign of its miraculous nature was that I was instantly cured of this particular habit. The last time I used profanity of any sort was before my conversion. I have not had a single slip up since. This was the change that people noticed in me right away. When I hear someone speak this way, even in movies, I experience physical pain in my ears.
May God grant you peace and healing.