I am a FT working mother to an 18-month old, and yes, your post sounds a LOT like me. I think you are normal, though it's frustrating to be "normal" in this particular case. I whine about feeling ambivalent towards work quite a bit in my journal :)
I went back to work at 3.5 mo postpartum. It was ROUGH. Are you still nursing? My son was still nursing 4-5x/day at 1 year, and did not stop nursing completely until about 14 months. Once I stopped nursing, I was amazed at how much my energy levels increased. In particular, when I dropped the early morning (around 5AM) feeding, which I'm pretty sure was when my son was doing the bulk of his milk chugging. He had been sleeping 7pm-5am for over a month at that point, so my sleep deprivation was not so bad, but I was still tired/low energy until we dropped that feeding. So that's one thing that might improve going forwards. I also noticed that the daycare bugs and sick days (for both myself and kiddo) dropped off around 12-13 months, though for us that was also getting towards the end of cold/flu season. Will be interesting (or awful) to see how badly we get hit this winter. Last winter I was getting sick at least once a month, and illnesses tended to linger. That has not happened in several months now.
Now at 18 months, my energy is more "normal" but I'll admit I am still much less motivated to bust my butt at work like I used to. I have tried to deal with it more by being selective about what work I get involved in...making sure I am getting my core job duties done but doing less above-and-beyond unless the work is interesting or obviously beneficial to my career. Sometimes it feels selfish (like I'm putting work onto others), but I would also say that I am ensuring my long-term performance stays reasonable by not overloading myself and then dropping the ball on something important.
I also rely a lot more on stuff like written to-do lists...I just cannot track things in my head like I used to. I find that if I take a few minutes in the afternoon to write out a few specific goals/tasks for the next day, I am more productive and leave work feeling like yes, I accomplished something.
It is a slow process, but I do think it is getting better over time. While I think I am ambivalent/underperforming now...when I look back and think about how I was feeling/performing 3-6 months ago, I see a big difference. I recently had my mid-year review and in preparing for that realized that I actually *had* done measurable work beyond the bare minimum, despite often feeling like I was barely doing enough.
One thing that has helped me a lot is negotiating some work from home days, this gives me a little "me" time while my son is in childcare (the time I would normally spend commuting), and I find I feel less stressed and more relaxed working from home than in the office. If you aren't already doing that, I would look into it. I only WFH one day a week most weeks but it makes a HUGE difference.