I'm so sorry to read that you lost Buttons. No matter how many times we go down that road of making a decision to help them cross, it never gets easier - whether it be one time or ten times. But we are blessed with the ability to help our animals be free from distress, whereas we cannot make that choice for a human loved one. I guess in a sense, the animals are more fortunate than we are.
I'm sure that Buttons thanks you for all your love and kindness. She's free now, and waiting for the day that she will be reunited with you once more.
RIP Buttons. You were/are loved!