Unless it were attached to wrath, I’d not think it mortal. However, it is bad language, and therefore a poor example to others whom should hopefully see the light of Christ in your life as witness to you being a faithful Catholic, so stop saying it.
As @Irishmom2 said, there’s nothing holy about it. And IMO, the simple fact that you pose the question indicates you feel guilty because you know it is wrong, plus you said you’re “trying to break this habit.” Just stop it.