Constant Sickness: Need Help Boosting Immunity

This is just a general post, but in the past year and 10 months I’ve gotten sick 25 times. I get sick about once a month and spend a lot of time out of school and work and it’s terrible. If I get sick again I think I’m going to go insane and lock myself away for ever. it’s really exhausting, i’ve been sick on Christmas 4 years in a row, I was sick on my birthday and my boyfriends birthday. I’m so tired of it, are there any tips boosting immunity and helping me not get sick? - I was tested for immunodeficiency and I don’t have it - I was tested twice for autoimmune disease, first was borderline second was negative :/