Beca, its completely normal, we all feel it. It never goes, but it does wax and wane. Every time they come up against a challenge that other children would just breeze through the guilt rises up again, but in between times, when progress is being made and things are quieter you do eventually start to have some degree of acceptance at least.
We also get to celebrate so many small things that our dcs do or achieve, but which other parents take for granted, but again these successes can sometimes be tinged with a bit of sadness, which opens the door to the guilt.
My ds was the same age as yours when I finally had enough of trying to get someone to take me seriously and went into battle. I know I will always regret not doing it sooner, but he has come such a long way in the relatively short time since then (he'll be 9 in a couple of weeks) and I have learned such a lot and found such excellent support - both on here and in rl that I don't tend to feel the same levels of despair I had in the beginning.
Nothing is set in stone and just because our dcs aren't necessarily going to turn out the way we predicted (although to be honest, whose children do? I for one know I'm nothing like the person my mother would have liked me to become) doesn't mean they can't live happy fulfilled lives and have a lot to offer the world.
As others have said, you have nothing to be guilty about, the experts should have listened. They had no right to fob you off. It was their job to respect your knowledge of your child as his mother, to listen, to hear, to 'know' and to make sure they exhausted every option finding out, instead of being egotistical naysayers.
I still get wibbly when I have to talk to certain professionals, because even after I was proven right, they still treat me like an over-anxious mother who can't possibly know anything of import against their saintly expertise. Not all professionals, some have honestly been fantastic, but certainly enough to still make me stress about being taken seriously. I am getting better, but think I still have a long way to go after having my confidence knocked so many times, despite actually being right.
I have recently come to the conclusion that guilt is a bit of a wasted emotion, far better to channel that emotion into the anger that enables me to fight for my ds.
I'm not saying it always works like that. I have my fair share of sobbing over my failures as a mother - had a good sob last night actually, after ds came out of school in a terrible state thanks to the way his teacher had (badly) handled a situation. But my feeling guilty isn't going to help him, so I have my sob, then take a deep breath, shake myself and keep on fighting - just like you will.
You are a super mum and you have done so much right - be proud of your achievement in producing and bringing up such a lovely, special little boy.