While I am no authority, this is my understanding:
Mortal sin, if committed, brings about spiritual death. However, the soul can be revived. Once the soul is revived, my understanding is that all of your merits are returned to your 'account', so to speak, although you seem to be asking about the work God has done upon you, not your merits. Depending upon the intensity of your contrition, it is entirely possible that you could not only be restored where His previous work is concerned, you could, in fact, move to an even higher degree of charity, etc.
While there are solid principles involved, how they apply in each case will vary.
Tell God you are deeply sorry and confess your sins when you are able. Trust in His love for you despite your weakness and/or infidelity. If you knew what I was doing at your age (and well beyond), you would thank God for His loving solicitude in your regard. Be truly sorry, but be at peace, too -- as much as that is possible. Do not dwell upon what is past, whether good or bad. Godspeed, my young friend.