Even if the humidifier itself is clean, any time the humidity gets above about 60%, molds have the opportunity to grow on everything else (furniture, bedding, walls, curtains, carpet, etc.). If you're dealing with allergies on top of a cold, it can make the cold last longer (because your immune system is compromised), and also make it seem like it's lasting longer because there's so much overlap of the symptoms.

Personally I have to be careful with echinacea, because it just pumps up my immune system indiscriminately and makes my allergies worse. Maybe try ashwagandha.

But if it's not just you - if everyone around you has had multiple colds - it sounds like maybe there are just that many different viruses going around your area, just bad luck.

Hope everyone feels better soon.