Just remembered today would've been my mums birthday.

[Moodykat]

And I can't fucking believe I forgot. Am in pieces now and upsetting DS1 by crying! Does it ever get easier on these days?

I'm so sorry to hear that you forgot your mum's birthday.
I understand why you feel so upset about it, but please don't beat yourself up about it. These things are so easily done.
Your mum would not hold it against you.
I'm sure she would say that it doesn't matter, especially as you are busy with your own children.
You remember her and think about her sometimes. That is enough.

In answer to your question, yes, it does get easier.
I still try to think about my mum on the anniversary of her death and on her birthday. However, as the years have passed, I've found that my remembrance doesn't always coincide exactly with the days in question.
That no longer worries me as much as it once did.