Is it just me or are people absolutely selfish?

Weeks ago I asked my friends to come out for my sister's birthday and they were all for it and promised to be there, and when the day came they said they had period pains and all that crap. They have a go at me for cancelling a couple of times because I have anxiety and get nervous about going out and sometimes I can't face it and I explain that to them.. It's only been twice that's stopped me from going out but they cancel a lot even though I asked them ages ago to be there.. Other friends aren't replying to my texts and when I ask if they got my message they just leave a question mark as a reply. I was also meant to be going dress shopping with my friend and on her Facebook just now she said she found a great dress online so I was like I take it we're not going dress shopping anymore.. I guess I sound really whiny but I am tired of it. I'm always there for my friends and put them before me most of the time but whenever I try and talk about something that is troubling me they quickly change the subject. What should I do? :/ I'm tired of this.