I was stupid.....

Recently, I haven't come here. It was because my stupidness!!!!
I have forgotten my address and password, got, can you imagine??!!!!

'Signed in here' Is very correct English, but it's more natural to say 'been here' 'come here' 'been on' or' or 'checked italki'  :-)

I was stupid.....

Recently, I haven't signed in here, due to my carelessness (stupidity)
I forgot my address and password. GOSH, can you imagine??!!!!

Carelessness is more appropriate.. :)

I think you mean Gosh instead of got......

But I can really imagine it too..... Though, I have not experienced it yet,,, Still it's good that you are back.... 

I was stupid.....

Recently, I haven't signed in here. It was due to my carelessness. 
I forgot my address and password,  Can you imagine?

Yeah we can imagine but good thing is that you are back, so Welcome back.