Like kryssi said, it can be any point during the first 12 months, please speak to a doctor if you think you have it, sooner rather than later xx
I think all us mums can resonate with what you are saying! Not every day is amazing, not every day is easy. If you have any concerns please speak to your health visitor, there should be no shame. I've recently spoken to mine about feeling low and yes grief about my old life and the things she has suggested I would have never had known could be done to help me feel better. Please seek help if you think it's needed but also know that you are not alone in feeling this way
Check to see if your local NHS has something called "talking therapies" you can self refer and they will do a phone assessment and recommend some options, always worth speaking to someone about it
It can happen at any point in the first 12 months! It's totally normal to miss your old life though and feeling guilty at not being able to do the things you could before.