Keep going laetitia - you're doing a brilliant job. This is the hardest part - you need to ignore visitors (I know not easy at this time of year particularly), get people to bring you drinks and food for you and just feed, feed, feed. I know its constant, we've all been there and you just have to ride it out - I promise its only for the first 2-3 weeks that feeding is so intense (then again during growth spurts).
As midna says, at the point you think you can't cope any longer, it'll start to get better - you are only a few days in, please, please bite down, ride out the pain of sore nipples and keep going - its sooooo worth it! I believe there are nipple shields and things you can get if it gets too unbearable - ask your HV about these if you feel you need them.
If you want to use formula too, I don't think its recommended to mix feed until Kenzo is a little bit older until your milk supply has settled down (around 6 weeks) and you can introduce a bottle without risking nipple confusion - although I know other posters on here have mix fed since the early days with success. Its all trial and error and you just have to muddle through and see what works for you.
Good luck hon - we're all here for you - post whenever you need us.
Love
Valentine Xxx