Sorry to hear that. I won't try to fault your experience, just state its not a given this is the case for everyone. I have been a vegan for 16 years now. I would say I am in excellent health. Each year I run several 100 mile mountain races along side training all year (running, swimming, hiking and biking), and will be attempting 258 miles non stop race for 5-6 days in the middle of the British winter 2023 (the montane spine race), so I must be doing something right. I very rarely get sick. I also manage a large team of engineers at work and have a young family.