Hi pinkelephants.
I think the advice you've had so far has been great.
I had an elective section for a breech baby who was scanning quite big. He wasn't that big in the end and over time I have begun to look back and wonder if maybe I should have had a go. The "what ifs", the regrets that his mode of birth is affecting how the baby I am carrying now may be born - it starts to niggle away at you, doesn't it?
I really think when someone has an elective section, despite the fact that you logically know it was the right thing, and would probably make the same choice again tomorrow if you had to, there's a feeling that you're somehow not entitled to have regrets. I think the suggestion to contact the birth crisis people is a really good idea. You have every right to feel the way you feel about your son's birth. Talking it through with others, even just to get your feelings outside of you so they're spoken and real (that sounds so corny but I hope you know what I mean), might go a long way towards helping you.
I wouldn't say you were lucky to have a section - lucky to have a wonderful, healthy, happy son, yes. Nobody is lucky to have had surgery, so don't dismiss your feelings. You can't undo a section once it's done, any more than you can undo a vaginal delivery. And like a vaginal birth, a section can have lasting effects on your body and your emotions, so don't ever feel that you don't have the right to feel the way you do.
Over time the birth becomes a smaller issue IME (DS is 3 now), simply because as they get bigger and do more it's a smaller part of their life. But I do think from what you've said that this could eat away at you and you will benefit from actively addressing it now rather than hoping it goes away in the future.
Wishing you best of luck :)