Switched from iPhone to Android. Friends can't send normal texts

Ever since I switched from an iPhone 5 to HTC One the texting and messaging has been a nightmare. Sometimes I'll text friends and they will receive texts but I won't receive there's because they will send it through iMessage. They don't know that I can't receive them until I actually tell them what's going on. For a company like Apple, who makes everything easy and simple, how come just switching phones makes it so difficult? I feel like there punishing me cause I switched. I've gone to my ipad, turned off iMessage, turned off the account with iMessage, Yet still my friends are accidentally sending iMessages instead of texting and I won't receive the text.

Is there another way where I can get it so my friend Willa ONLY send SMS since I'm no longer on an iphone? It's weird because occasionally it will fix itself too! But then it will go back to imessage. Can anyone help me ASAP?