You aren't a loser. You made an error in memory... so what? I don't mean that offensively, because I do know how horrible it feels to always pay on time and then something slips by... but really... from what I hear, "everyone" does it.
It happens. I agree, if there are 2 of you capable of double checking these things.. what happened to the "double" part of that rofl.
Go easy on yourself jmo... yeah, it's a biggie on the interest thing, but it isn't the end of the world.
You sound like you are doing all you can do...

how can you do more than that?
TC