Question numberrrr 2!!!

In chapter 43 in the first couple pages, (352-354), Pip is saying that since his benefactor is a convict, he can no longer be with Estella (even though he really wasn't from the beginning). I understand the reason for him to feel this way, because he thinks she won't want him because all his money is coming from a convict, but why would he even tell her where all the money came from? Can't he just keep it quiet so that Estella will think it's his money? Maybe it's because he would feel guilty, but I still don't understand it completely. If he told her that it came from a man named Magwitch, how would she know he is a convict? I don't think she will even care where the money came from, so why does he think she will care so much? And why does he need to tell her where it came from in the first place?