Hi,
I totally understand where you are coming from. My DD is also 4 months and all seems to be going ok too although I am forever worrying if I'm doing the right things by her! It sounds crazy but there is so much information about babies sleeping, eating, growth and development I find it hard just to relax and enjoy her!
I too have seen myself change into someone different...yes, more nagging, more uptight, less relaxed when I used to be pretty chilled out! I remember thinking when I was pregnant, that I am going to be the crusiest Mum, just let things happen how they should, not worry about little things, not change too much about my life but having a baby is a life changing thing and until you have one, you can never really know what it means.
What has helped me is to put it all in perspective. I leave myself little notes on paper stuck around the house, that remind me what an important job I have now because being a mum is a job and when you think about it, when you take on a new job in the workforce, it takes a long time to learn all the ins and outs and get used to your role.
I tell myself to stay calm and relaxed, that I'm still the same person but now I have another important responsibilty and I'm no longer the most important person in the world, my daughter is. I love her to absolute bits.
I try not to miss my previous life but think of the wonderful things that I can look forward to as my DD grows. Some days it's very hard!
Everyone always says "They grow up so fast" which is so true. I make myself remember that everything else will fall into place in time but my baby will soon grow up and I don't want to look back and realise I wasted time worrying.
Sorry for the long reply...I'm sort of relieved there are others who feel the same!