I think babies understand our sadness on a simple level because they are so in tune with us. But I think it is most important that A, you are alright, and B, that as long as baby knows that he is very very loved it will not have a massive impact.
The advice is right, maybe this is just a one off, we all have days like that. Being a new Mum is the most overwhelming thing in the world, However wonderful. But talking to someone is very helpful, whether a friend, family member, or your HV.
If it is more than a one off, remember to be kind your yourself, you are doing an amazing job, and if he smiles at you, he obviously thinks so!!
So many of us go through something like it at some point, so you are in no way alone..
Mine are 11 and 6, and it still takes my breath away .
Give yourself a pat on the back, and if you will accept it....a big {{{{{}}}}} from me. I hope you feel better soon, let us know one way or another, it is why we are all here...