Hey. Good reply; seriously. I'm about three times your age and have worked with men and women in your age group at a major university, so I have some experience with which to gage the replies I might receive. Yours is impressive.
Your's is a very complicated situation; probably beyond the scope of r/dogs and online comments.
As to your guilt and shame. You should be thankful for those because if you felt none of that it would point to difficulties far greater than what you're dealing with now. Your feelings show that you're human.... Good place to start, as opposed to being a psychopath.
16-17 can be difficult years. You're not the first kid by a long shot to hit or abuse a pet. We often attack those less powerful than ourselves even when our anger is towards someone else (who is too dangerous to attact directly). As you have, frankly acknowledge to your self that this happened, it was wrong and that you're absolutely determined that it will never happen again .... THEN MOVE ON. Not by forgetting... but by forgiving yourself. Whether Reddit forgives you or not is beside the point. The forgivness YOU NEED comes from within: to accept that you are human and you made a mistake.
Beyond that, don't keep beating yourself up with this. It doesn't help the dog you mistreated and it makes it difficult for you to move on with a positive life which I think now that you've shown you deserve.
Oftentimes when people hold onto guilt or shame and won't let go, it's because they've come to believe that by showing on-going remorse ....for ever and ever and EVER... They are somehow showing how seriously sorry they are: "Look at me. I'm SO sorry I can't even stop BEING sorry." This is not productive and it impresses no one. Accept your guilt then let go of it so you can move on with your life and have the opportunity to teach others that you come in contact with a better way of treating their less-powerful friends.
You've learned an important lesson early in Life. Don't mistakenly now sabatoge the rest of your life in overwrought depression. It serves no one any good purpose.
Your counselor, nor any other, can ever tell you that you are no longer capable of your earlier conduct. Human beings are capable of many terrible acts. Your questions need to be along the line of impulse control. It's one thing to feel rage or anger; to act on that feeling is the damaging part.
I'm not clear if you're living with your GF or not. If you are, and you decide to go ahead with getting a puppy, your counselor can help you set up a safeword signal with your GF for use in the event you ever feel that you're 'about to lose it' with the dog. You would use the safeword and she would take over the situation with the dog, whatever it might be, and you distance yourself (whatever that might mean at the time).
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Just a few thoughts; I wish you the best.