Hello and welcome to Beyond Blue. I'm glad you found your way here.
What you are describing is the classic reaction to loss. That probably doesn't help much but I want you to know you are walking a path those others have used and you will come out the other side.
May I ask if your mother is still around. If so I imagine she is equally as distressed as you. One of the helpful ways of managing your grief is to talk about your dad. Perhaps you can arrange for a few family members to meet at your home and to bring something they associate with your dad. Talk about him and remember the time you had with him. Some will be funny, some sad, it doesn't matter. You will all gradually realise that the pain has receded a little.
You will never forget your dad but the memories will help to see him as your much loved dad. Play the remembrance game, "Do you remember when ........ It may sound morbid but it is very good for you to talk about your dad and what he meant to you. The feeling he is going to walk in the door or you just saw him in the shop down the road is very familiar. I used to look out of my window and expect to see my dad coming home from work. I live in Brisbane and my dad was in the UK. How can you figure that out. It was the same when my mom died. I did not get to see her before she died, only attended her funeral.
I would think about something and pick up the phone to call her before I realised what I was doing. This is how powerful your grief is. You will know dad has gone and you will know he went peacefully. As you say 55 was no great age and he should have had longer.
It's the same with seeing him in different places. You know it's just someone who looks a bit like him but it still reminds you and still hurts. It is a stressful time and you have a young baby to care for as well.
Give yourself a specific time each day to think about him and mourn. Half an hour is enough. When you find yourself thinking about him remind yourself you are going to have a time later that day to really think about him. It will help.
Grief takes as long as it takes. I know you are unhappy but gradually your dad will fade into the background. Not forgotten but there as a gentle presence. Talk to him if it helps. I find it useful to talk to my parents even though I will not see them again. It helps.
Please come and talk more here. It always helps.