@hrafn I can understand why you feel that way, but you aren't failing him. You even have the courage to share this with all the members of the forum. I see a concerned owner. I see a loving person.
I had a goffin who was a special bird. She had splayed legs as a chick, I don't think she was raised properly as she was a very nervous bird. Her hatch date was in 2006 and we ended up with her in 2007. She really struggled with hormones. She pulled all the feathers out of her face, using her foot. It was traumatic and horrifying for the whole family. We went to the vet several times in the course of three months. She was healthy physically, but clearly not emotionally. She went through a phase when she was in a cone (this was 2007), she was on antidepressants... which was hell. We got her off that treatment. We found some of the treatments were only adding stress, and actually harming more than helping.
She ended up getting over it on her own and stopped pulling feathers. For her I think it was just the intensity of her first real hormone season and she was frustrated and confused. She was a bird that was out 4 hours a day, when she joined our family. We had toys for her, played with her, love her deeply, we didn't offer any nesting areas.
The thing that really helped was toy rotation, reducing stress (i.e. the cone only made her more stressed), increasing sleep, giving her her own room to sleep in so we weren't disturbing her (she used to sleep in the living room), limiting physical touch during the spring, encouraging foraging. We worked hard to make a clear flock bond. She never had shown sexual behaviour towards us. She still nipped the tips of her feathers. After that first bad moment, she never went back to that severity. It isn't easy and it is heartbreaking. There is much better information available now and much better breeding practice, not all birds had that advantage.
Foraging for food will help reduce the amount of available time Taco has for self harm. Stay patient. Stay loving. Do your very best to make Taco's life more challenging and stable. It is a very difficult problem to overcome. Taco may never change his ways.