If you feel they came with the vegan diet, they may be linked to a reaction to citric acid if you have increased fruit consumption that are high in citric acid. Try milder fruits like bananas and grapes for example.
A B12 deficiency is unlikely unless you've been vegetarian long before you went vegan, since it takes between 1-30 years for clinical B12 deficiency symptoms to appear in B12-less vegan diets. Do in any case always follow the standard B12 integration, 2000µg (2 Nuggets, e. g. from Solgar) once a week, irregardless of ulcers.
Improving you diet does not have to be expensive if you "cook from scratch" a good whole wheat bread with a lentil spread and a kale/carrot salad on the side is already a highly nutritious dish. Get a pressure cooker for quick legumes-based dishes. Soak overnight.
Drop your current toothpaste and brush your teeth with some salt only.
Drink only nonprocessed drinks like water, water/juice mixes, coffee, tea.
Make sure you get enough sleep, improve your efficiency of your day to day organization to carve out more time for sleeping. Deal with stress levels aggressively. Don't "swallow" conflicts, do things that brake your routine, or do the same things slightly differently, have sex/masturbate if that is legal where you live. Don't let depression rule, remember that even if this world totally goes to shits, existence is still magical, unique and mysterious and how wonderful it is that YOU exist.