Look, much of the time the right thing to do is fairly obvious. (Don’t smack your sister for borrowing your clothes, do pray for your enemies, don’t flip off the other driver and start screaming at them because they cut you off on your way to the monastery retreat, etc.) Sometimes it’s not, and “least bad” is as good an option as you can find. And sometimes you just choose the wrong option, whether it’s because you were mistaken, ill-informed, tempted, rebellious, hurting, confused, or just human.
Of course you feel terrible. Sin is a wound, and wounds hurt. And they do damage if they’re not addressed, so of course you have to deal with things. (They also itch when they heal, so the path from here to the straight & narrow is going to be unpleasant at times. Just a heads up.)
First and foremost, talk to your priest. Go to Confession, repent, pray, cry. Then pick yourself up and keep striving.
Fall and get up, fall and get up. This is the Christian life.
Repentance is hard. Moving forward is hard. Forgiving yourself, though, can be hardest of all.
Sometimes berating yourself is used as a way to get out of actually dealing with the underlying problems, and that’s a copout. The fact that you have sinned does not mean you somehow get a pass on working on your spiritual life because you’re now damaged beyond repair (FYI: you’re not); rather, the sin has made your struggles pretty glaringly obvious, and now you get to do the hard work.
Also, this will not be the only “something terrible” you do. Cf. being human.
As far as the hows of dealing with your situation, well, that’s a definite AYP* issue. But you do have to deal with it, and you do have to move forward.
You’ll need to make adjustments. Figure out what works and what doesn’t with your boundaries, and how to protect yourself from making the same lousy choice again. How can you keep yourself out of that situation in the future? How can you prepare yourself to make the hard, better choice next time around?
And when you fail again, re-evaluate again. Ask yourself the hard questions. Talk to your spiritual father a lot, pray, talk to people you respect, read nourishing things, and hold yourself accountable. Change isn’t easy. Healing isn’t easy. I mean, crikey, living isn’t easy.
Sin is a wound. The church is a hospital. And the Great Physician is not going to abandon you just because you screwed up.
Have mercy on me, O God, according to Thy great mercy; and according to the multitude of Thy compassions blot out my transgression.
Wash me thoroughly from mine iniquity, and cleanse me from my sin. For I know mine iniquity, and my sin is ever before me.
Against Thee only have I sinned and done this evil before Thee, that Thou mightest be justified in Thy words, and prevail when Thou art judged.
For behold, I was conceived in iniquities, and in sins did my mother bear me.
For behold, Thou hast loved truth; the hidden and secret things of Thy wisdom hast Thou made manifest unto me.
Thou shalt sprinkle me with hyssop, and I shall be made clean; Thou shalt wash me, and I shall be made whiter than snow.
Thou shalt make me to hear joy and gladness; the bones that be humbled, they shall rejoice.
Turn Thy face away from my sins, and blot out all mine iniquities.
Create in me a clean heart, O God, and renew a right spirit within me.
Cast me not away from Thy presence, and take not Thy Holy Spirit from me.
Restore unto me the joy of Thy salvation, and with Thy governing Spirit establish me.
I shall teach transgressors Thy ways, and the ungodly shall turn back unto Thee.
Deliver me from blood-guiltiness, O God, Thou God of my salvation; my tongue shall rejoice in Thy righteousness.
O Lord, Thou shalt open my lips, and my mouth shall declare Thy praise.
For if Thou hadst desired sacrifice, I had given it; with whole-burnt offerings Thou shalt not be pleased.
A sacrifice unto God is a broken spirit; a heart that is broken and humbled God will not despise.
Do good, O Lord, in Thy good pleasure unto Zion, and let the walls of Jerusalem be builded.
Then shalt Thou be pleased with a sacrifice of righteousness, with oblation and whole-burnt offerings.
Then shall they offer bullocks upon Thine altar.
Don’t give up.
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* Ask Your Priest.