I scarce believed thou couldst be gone away ; (line 2)
The speaker starts out in a state of shock, but we don't know what type of shock it is. If his lover has died, then he is having trouble imagining a world without her in it. He doesn't know what to do now. But we think it's more likely that she is alive, but separated from him for a day, so his shock and disbelief are more complex and difficult to understand.
Tears drown'd one hundred, and sighs blew out two ; A thousand, I did neither think nor do, (line 5-6)
The speaker falls into a "woe is me" complaint that seems typical of many a young Romeo who wants to sound dramatic. He weeps and sighs a lot, which are clichés of melancholy and despair. He's not trying to differentiate himself from all the other lovelorn guys out there.
forgot that too. (line 8)
The speaker's stages of grief conclude as he forgets what he was grieving about in the first place. He has been spiritually paralyzed by her absence.