Its like in the Imitation of Christ when the story of the man who is terrified about his eternal fate and is prostrate in a church and sighs "if only I knew if I were to be saved" and hears the voice telling him to act as if he were to be saved, or something like that.
so no need to excessive fear, but always should fear the justice of God and that you are on the road to Hell.
never should console yourself that you will not be damned. Because even if you are in a state of grace until your last breath - you can still be damned for eternity if you entertain an impure thought in your last agony, etc.
that's why: "make out your salvation with fear and trembling".
what I mean is that this life is so uncertain and we can't know if we are pleasing or offending God in many things, so we need to rely solely on God and not our own perceived merits, which may not even exist. when we are judged, how many of what we thought were our good works will turn out to be of no avail since they were done with impure intentions, etc.
a question for non-sedevacantists though: If one were to want to discern a vocation but entering the SSPX seminary was out of the question for reasons beyond one's own power, would entering the FSSP seminary be advisable? Question aimed mainly at SSPX parishioners.