Emily, you need to pay attention to what you said here:
I rarely get sick, even colds. I have only gotten injured from training once in two years of high-volume training. I've been overtrained once, but it took a lot to get there and in general my health and recovery is very good.
You are getting "sick" quite often now, it seems. This means that in general, your health and recovery are NOT good now.
Recovering from overtraining, especially that done from heavy, high volume training, is not as easy or simple as people make it out to be. Once is all it takes for some people to really begin a downward spiral that they never take the time to fully recover from. The fact that you say it "took a lot" to get to overtraining also means the converse is likely true--that it will "take a lot" to truly get out of it.
The saying I use for situations like yours: "My car was working FINE...until it BROKE." Everyone's health was better before it got worse. The things you have chosen to do (or chosen not to do) are not supporting your body in moving back to a better state of health, that's the reality of what is happening in your situation.
Last thing. I have people come into my office with things like rheumatoid arthritis or cancer, who say, "my health is great, I never get colds", and yet they are sitting in front of me not realizing that they have massive dysfunction in their immune system. Don't be one of those people. Deal with this early and correctly or it will dog you for years to come.
I'd suggest you find an ND in your area or I can help you run some labs that will help to give us an idea of where to pursue first. PM me if you are interested.