Very similar experience to bakingtins here. Wanted to add my encouragement and support because I rememebr the first few weeks after diagnosis were very, very hard - but it gets easier. The guilt is hard because you feel it's your fault - please try not to feel that way though! You are doing an amazing thing feeding your baby and once you can calibrate this dietary stuff it is totally doable and so worth it.
My ds started having bleedy nappies at 5m and after weeks of appointments but no clear diagnosis I was desperate and a young registrar at the hospital suggested cutting out dairy to see if it worked. In the end I cut out dairy, egg, soya and beef and even then it was a good 6 weeks before the bloody nappies stopped. I then gradually reintroduced each thing with the support of a dietician ( had to be pretty insistent to get referred on the nhs though!).
Once we had it sussed ds was so much happier. The first few weeks were really hard as it feels like there is literally 'nothing' you can eat. But once you get your head round it, it's doable and actually a really healthy, tasty diet. My top tips below!
Cook and bake - there are great recipes on the Pure website (Pure Sunflower in the butter section is your friend, it is great for cooking, spreading etc). Get a dairy free cook book for tasty cake and treat recipes - you need treats! Co yo coconut yoghurt is great, vegan chocolate is lovely (eg booja booja, mini moo) and nakd fruit bars (esp the choccy one) are great to take with you everywhere (have one in your handbag so that when you go for coffee and cake you aren't sat there hungry while everyone else tucks in. Ditto take a little jar of oatly/coconut/almond milk so you can always buy a black tea or coffee and add your own milk).
Thai curries and recipes are great as they use coconut milk and are super nourishing and tasty, you can make a yummy rice pudding with coconut milk and add mango etc for dessert.
Ready meals - Waitrose do two Thai curries that are totally ok to have and their Piri piri chicken is also free from all of the above and very nice.
You can make your own pizza, just skip the cheese and use rocket to top it instead. ditto risotto, top with bacon to add flavour without needing cheese or butter.
Lots of super cheap things are actually ok - hobnobs, digestives, chocolate shredders (!) and the cheapest bread is often soy and dairy free. Birds custard powder is also fine if you make custard using milk substitute (Oatly is good for this).
The hardest thing is when other people cook for you because you feel so rude interrogating them about ingredients or saying no to stuff they've made for you. Unfortunately most people won't understand and will say 'oh its only a tiny bit of butter!' Unfortunately your body/your baby's body doesn't know the difference between a lot and a little and will react no matter how much the amount.
I absolutely loved breastfeeding and fed ds till he was 14m - he's now totally allergy free and a strapping 3 yo who eats everything in sight, even ice cream, eggs, cheese etc - all the things I thought he'd never get to eat!
Very very good luck with it op!