As a single mom, I spent years with no one to celebrate me on Mother's day, birthdays or Christmas. I had no family around also so I was totally alone.
When the kids are young, they either have no knowledge of these celebrations, or can't take themselves to buy you a present or 'do' anything for you. There are no DHs or DPs to do this for them either. So lots of moms go through these occasions celebrating their kids and others, taking themselves out, buying themselves presents or just letting the day pass.
I have sometimes been in a relationship and have had a bf / dp to celebrate me. I'm currently (totally happily) single and my kids have only just got to that age where they have become aware enough to make me a card and buy me a little present with their pocket money and I feel so grateful.
So if you're out there, being a mother but have no one to celebrate you, I wish you Happy Mother's Day and lots of good times to come.