really hard on myself for my son not reaching ALL milestones

As a first time mom, im so upset at myself for not knowing more and being more knowledgeable. I had a mom at a baby party recently say the following:

“Your son is 14 months and isnt walking??? Or able to hold a regular cup to drink milk??? Hes still on formula??? Whats wrong with him?!?”

First of all, im a single mom doing my BEST. Hes taking his sweeeeeeet time with walking, currently pulling up to stand and cruising. I didnt transition him on cow milk yet because of very bad constipation he had, doing that now though. He really hates using an open cup and still hasn’t mastered it. We’re practicing 😔

I just feel like SUCH a failure and want to cry. These judgments are eating me up alive.