Hi Buster. Similar here. Went from no children to one adopted one.
It's a massive period of transition, adjusting to full on care of a small child, the physical demands and quite often boredom and isolation, huge changes in your routine. Coupled with the dawning realisation that your own life is now changed beyond recognition.
I've spoken to some first time birth parents who have felt the same, so it's not unique to adoption. However I think there's much more pressure on adopters to be all smiling, singing and dancing because "Hey, you got what you really, really wanted and must be over the moon with gratitude".
Many people don't understand how difficult the transition to being a parent can be, regardless of how you came to be a parent. It's bloody hard for many of us, even if it's what you really wanted and fought for.
Also, adopted children have usually had some trauma in their background which can make them much harder to cope with and bond with. Their needs can be very much higher than those of other children. Their abilities to form attachments are also quite different.
My daughter was so very difficult to care for in the early days. The defiance and challenges came by the hour. I used to get her up and care for her in the mornings, put her up for her after lunch nap and cry for an hour. Then get her up, care for her, put her to bed in the evenings and spend hours crying.
I used to think she actually hated me (my rational brain knew this wasn't possible but my emotional brain felt differently). It is now my understanding that for many adopted kids mummy is the enemy and must be proven to be the enemy because any time now she is going to hurt you/neglect you/abandon you, just like the last one. So mummy must be tested over and over and over again to make sure she stays.
My daughter has tested me in ways that have nearly broken me, many times over, but she never behaved like that with anyone else around us. I now understand how terrifying it is for a traumatised child to build a primary attachment, particularly with a mother figure.
This may explain, to some extent, why tour lo seems easier for your partner. Maybe, maybe not, just a thought.
It's a life-changing and emotionally fraught situation.
Keep the routines, keep acting like everything is calm and predictable. Talk honestly to people about how you feel but be wary of those who seem to not get or judge you. They can cause you terrible emotional harm. Sally Donovan calls it the 'Adoption Klaxon' if I recall correctly. The klaxon that goes off in your head when those around you, both family/friends and professionals, are not listening properly or trying to fully understand you. Don't waste time or energy trying to explain yourself to them.
The process of adapting to parenthood, coupled with a traumatised child is an ENORMOUS task and took me years, literally. But, it can be done and you can eventually feel like a competent and even 'elite' parent a few years down the line.
It's like eating an elephant, manage it day by day, bite by bite, and eventually the task will be completed.
Take heart Buster and take care x