Like you, I have the same issue, I get colds and flus all the time, someone sneezes on me and there I go. Like you, I have also tried everything for years and years, too many to think about, anyhow I found out about 10 years ago that I have primary immune deficiency, lack of producing adequate antibodies. Have you had your antibody panel checked? It might be an idea. So I have tried antibody therapy numerous times but I always go off and do my thing in the natural world but now I am back getting them again. However, I never ever give up my healthy lifestyle, I continue to do all and everything but if the antibodies aren't there they aren't there...not much choice.
I can tell you that if I have a cold/flu in the making or full blown that after I get the IVIG that my body kicks in and flushes it out very well within a few days. Otherwise, like you, I think the vitamin c is the most effective so when you say that I think you might have deficient antibodies as well...it's a thought and at least checking out. I typically take 2000 to 5000 mg a day. Not enough for kicking out something but enough in the scheme of daily intake.
Eating well and all that is so important but it won't clean it up completely, unlike people who have an immune system that is struggling but not deficient they can build it up really well to get their immunity and find peace. In my case that isn't true and it sounds like you might have something similar going on.
Hope this helps.