What was Big Willy Shakes going for?
Boo-hoo. Woe is me. My life sucks. King Lear is the king (see what we did there?) of pity parties. He knows how to feel sorry for himself, for sure. Here, he's complaining to us because he thinks others have done things way worse than he has.
Sure, he's done wrong sometimes (what with all the "sinning") but not as much wrong as his daughters have done to him (that's the "sinned against" part). It's just like when you were a kid and you fought with your brother. Your mom would get mad at you, and your only response was "but he started it." Lear is pretty much saying the same thing. His daughters are the ones who are bad; he's just the victim.
This speech is the ultimate parent whambulance. Lear is saying his kids just take, and take, and take… until he's got nothing left. The sad thing is, he's not exaggerating. His daughters are cold-hearted and cruel to him. The guy is literally standing outside in the rain, with nowhere to go.