No Nut Challenge gone wrong

This is a real story and im still so ashamed of myself. Im a 21 years boy and study psychology. So im writing this to cope with myself in hope it will help me to feel better about what i did and i wont do it again.
So this is what happend. I have my own flat payed by my parents and i own a Dog. Her name is Lucy and she is a rhodesian ridgeback. I had semester holidays and therefore i had mostly nothing to do. Good for lucy because of her race she needs a lot of Attention and exercise. I also wanted to stop Masturbation for a while because all my stupid friends were always trying these No nut Challenges. I think its stupid so i never did it but this Holiday i wanted to try it Just to figure out why everyone is doing it. Usally im masturbating every day once or even twice, so it was quite difficult for me to begin with.
The first day was okay but the second was very horny. I started the day with an ungodly boner and everytime i just even thought about sex i went hard again. So i did a lots of running with lucy to stop thinking about it. In the third day while Jogging i noticed that lucys Vagina was really swollen. She must have gone into heat since i never spayed her but i would not let her run around to get pregnant. I thought No more about it and didnt had any wrong thoughts about it. The next night was horrible. I had a constant boner and was going to jerk of but luckily before i could i did fall asleep.
So now we have day 4. And this is were things went wrong. Let me explain.
I woke Up just as normal, not even having a boner in the first place but immediatly caught lucy with my eye watching her licking her pussy. I dont know what it was but it reminded me of Masturbation so i turned my head to the side and got semi errected. I was done. I was holding my face and thought that this would be it. I would blow a load today. So i decided to get my Phone and watch a porn. But the Moment i turned Back lucy was standing infront of the bed. Her face was directly in my face and then she turned around showing me her big swollen pussy. And then she teased me. I watched her vulva just a second when her upper body felt to the ground, basicly inviting me like a Male dog to mount her.
And here is the most embarassing part - a shiver of pure wild extatic hornyness trembled through my body. My Penis went rock solid and started throbbing. Uff....i turned my face immediatly back because i was also shocked about myself. I never had feelings like this for my dog but there it was. I looked back to her and then sat down in the bed. The hornyness didnt Go away. I needed to cum now but that pussy...it was brown and round and big and a little wet too. And the offering was to much for me. I wanted to Stick my penis inside Her. She even crawled backwards. Her hips were up and at the exact same hight as my dick and she was pushing her vulva closer and closer. I started to sweat because i went more and more excited as closer she came. One second i had a normal thought again that i should stop it but then her pussy touched the tip of my boner and i was not holding back anymore. I needed to cum and this was to erotic. I wasnt sure what to do so i slowly pushed my tip into her soft swollen pussy when apparently she stood up and pushed herself backwards. She was sort of thrusting my dick into position with only two Backwards movements. I moant in agony her insides were hot as hell. And My Penis went her pussy up in a very strange angle like 60° and it felt like i was stucked after i pushed myself completly in. I was completly brain dead at this point. For a second i thought NO Pulli out, so i stood up and pulled back but it felt to good i had to moan and immediatly pushed myself Back in. I was already in the edge and i needed to pump my load inside her. So i thrusted as fast as i could and exploded my balls deep down in my Dog.
I was pumping so hard and the releave was awesome. I would have never thought that Ni Nutting would make orgasms that tense. But here was i - balls deep in my beloved lucy.
I pulled out and had immediatly regret. Wtf have i done. I fucked my own Dog. I still cant comprehent this fact and im not even think about doing it again. I dont think dogs or lucy are hot or make me Horny in generell. I still feel so much regret and embarassment. Im also scared of myself now. It seems that If i cant Blow a load in a regular base i get Mad and fuck everything.