Sounds like you're doing lots of the right things. However, because you've been eating this way for 18 months and been sick about 75% of the time something needs to change. The first thing I would do, personally, is get off of the multivitamin as it's just another chemical (most likely). Here's some other questions to consider:
Have you traveled anywhere in the last year?
Have you done any cleanses?
Do you eat meat?
How much and what kind of water do you drink?
How is your mental state? (positive attitude?)
Do you use lots of synthetic cleaners/detergents/cosmetics?
What is your height/weight?
What is your energy like throughout the day?