I feel like I’ve messed up

I have a 6 month old male Labrador X Bernese mountain dog who is anxious and barks at things he’s scared of. The neighbours are doing something in their garden that sounds like scraping rocks and he won’t toilet in the garden because of it. He barks at pushchairs/ strollers, trollies/ shopping carts. Idk if this counts as reactive. But I feel like I’ve failed him and as a result messed up his temperament making him unsuitable for assistance dog training. I don’t know what to do. It’s plummeting my mental health. He’s neurotic and his first port of call when he doesn’t like something is to bark, so if I take too long to give him a treat, he barks, we’ve been standing in a queue for too long, he barks, he’s scared of something, he barks. The breeders picked him out because he apparently had a sound temperament so I feel like I’ve messed him up in a way that I don’t know how to fix.

Everything is a challenge and something to overcome with him. I feel like everything is snowballing and I’m in way over my head.