I am no doctor but guess the situation has gone back to extremes as you just had a baby- that is a huge physical and emotional adjustment and you are having to be there for a baby, so the food is probably an area where you feel you can control. However your baby needs you to be strong and well. They need a lot from you so you need support for you too.
The truth is there is no 100% fail safe advice on diet, it's best just to follow a few simple guidelines (the NHS website is good) on calories and macros.
By all means work towards cutting down ultra processed food, but having some ultra processed food won't harm you. A diet fully free of UPF can be hard work and quite restrictive.
It is a good thing to think about and get educated about though.
I'd say for now, allow yourself more of what you want even if it is 'unhealthy' as it is better to have the energy. And practise not judging yourself for it.
You could try one of those recipe boxes like Gousto for a middle ground.
If you do want to introduce it gradually, have you thought about joining some facebook groups like Reducing UPF where you can share plans and get ideas. You might decide some is and some isn't for you.
Eating issues sometimes become self fulfilling prophecies, the more you fear food, the more you attract comments which then stick, reinforcing the behaviour or the fear of changing it depending on how negative or positive the comments are. The best thing is to let go of some of the control and give yourself grace/permission to indulge/gentle encouragement.
80% good 20% treat is absolutely acceptable (my personal trainer who is recovering from an eating disorder taught me this).