I'm so sorry to hear how this has been affecting you and your boy.
I can't relate to the shadow chasing, but my girl was very reactive to birds and would bark/lunge/generally lose her mind whenever she saw one and I felt like it would never get better. Birds were her obsession and like you, I started dreading walks and couldn't see the wood for the trees.
Our gundog trainer suggested training a 'look at me' cue whenever we saw a bird - so ask her to look away from the bird and up at you and then 'yes' and treat. I was spectical as there was no getting through to her when she was in nutcase mode, but I found that if I got in there with a 'look at me' before she went to bark/lunge then it was so much more effective. There were quite a few times I missed her signs and the barking/lunging happened, but I found that if I walked her away and sat her up at a greater distance then she would calm down eventually (9 times out of 10) and I could try asking for a 'look at me' again. If she didn't calm down, that was the end of the walk and we just went home. After months of doing this consistently on every walk, she will now look at me when she sees a bird without me asking (on lead, off lead is still a work in progress!) I guess this is just another way to redirect behaviour and ask dog to do something else before it gets fixated. This may not be the right cue for your dog, but something similar might work?
I wonder if gundog training might help as even if you aren't planning on working your dog, they give great games and advice for tiring your spaniel mentally, which is so much more effective than physical exercise. There's lots of advice out there about upping exercise for dogs who are showing frustrated behaviours and that may work for other dog breeds but honestly, I don't think that applies to spaniels. It just seems to wind them up even more and make them less likely to listen to you. I found playing mental games like 'go find' and short retrieves would knacker my girl out more than hours of running when she was an adolescent.
I agree with everyone else about going back to the vets. If you have a good vet, then they will often suggest training or herbal supplements to try first before going down the medication route. But if medication is the conclusion then please don't feel bad about it - you have to do what is best for your boy, and his quality of life. I look after a rescue staffie who is on anti-anxiety medication as a last resort and it has made a real, positive difference to her life. As well as reassurance for her owner.
On a side note, I've never heard about dogs getting stuck with baby brain and am a bit spectical of that too!!
Good luck to you both. I hope you get through this tough time.