My dog turns out to be aggressive to dogs. How do I train for something like this?

We started going to puppy school at 14 weeks. First he was extremely anxious just being on the training grounds.

I got adviced to stay out of the way of other dogs as much as possible during class until he would be less anxious. We worked hard and with a lot of patience, he became less anxious. Sometimes even sniffing other dogs on the lead.

But since we transferred from puppy to obedience class in January I don't feel he has learned anything new in 3 months because he is always so distracted by the other dogs. The last few weeks a dog that has been advised to get behavioral therapy outside of obedience class, has been falling out toward my dog a few times. And I have the feeling my dog has become even more stressed during class.

I was actually taking the classes in the hope of him getting used to being around other dogs from a young age, but I feel like by wanting to do the right thing, I actually did the opposite. So I decided to quit the classes and try to arrange some one-on-one positive interactions for him.

Today we visited a gentle dog of similar age and breed as mine (10 month old doodle) and it was a complete disaster. The other dog was quite excited and loud the first few minutes, but really gentle, just wanting to sniff mine. While my dog was really anxious barking the whole time, trying to get away (they were both off lead to have the choice to approach or back away) and when he came closer, even snapping at the other dog. I knew he was a bit wary of other dogs, but I had never seen this side of him before. And he didn't calm down. Yes, he would lie down, but the moment the other dog moved, he was in defense mode again.

So turns out by going to the school to socialise him, I actually failed miserably at it. And now he is a 10 month old adolescent who is aggressive to other dogs. I know we are in for an awfully long way back now, but I was hoping to get some advice or resources (classes/books/online trainers/behaviourists). How do I approach this the good way and how do you set up a good and safe meeting between dogs?

I would appreciate any help, negative comments about how it's all my fault can be kept to yourself.