I lost my birthday present from my boyfriend...
How did that happen?  And do you think he'll understand?
I'm sure he's not going to like me no matter what.
I'm sure that you showed a lot of gratitude for the nice gift that your boyfriend gave you, and that this was a simple accident. However, if you keep losing things, you should try to be organized better and value your items more.
I hate being grateful.
I don't quite understand why you hate being grateful, but I'm sure you love receiving gifts and your boyfriend's affection.  So just let him know how much you love those things, and if you are honest about what has happened today, he'll understand and forgive you.