I have felt this on and off lately, and I think a lot of it is hormones, as well as some fear/guilt about adding another child to our family (even though this baby was very wanted and tried for).
I have dealt with depression and anxiety, so I know that for me
, some simple cognitive reframing can help quite a bit. I keep a gratitude journal, where I consciously focus on the joyful things in my life, and meditate on those things, rather than the things that feel so heavy (but which are very small in reality). There are many other simple reframing techniques, including visulizations (breath out the dark, breath in the sunshine) and I make sure that I get outside when I am feeling low. i also have to make a effort to reach out to friends, even though I am an introvert by nature. Isolation only makes it harder for me.
There is no shame in being depressed, and you do not have to justify it to anyone. If you are feeling low, tell your partner, and ask for help. It may just be giving you time to walk by yourself, or it may be helping you make a plan for how to fight back. You are not alone in this.
I'm sorry to hear about your dad. My mom passed away almost 8 years ago now, and around her birthday and the time of year that she died, I still feel the loss of her intensely. There is no timeline for loss.