Dorothea and Gwendolen, despite their different personalities and mental abilities, have a few things in common. They lack an authority figure, a kind of guidance. They have neither a keen sight, nor a set of principles on which to rely to choose a man (unlike those Jane Austen characters, except Catherine). They're mistaken about the men they marry, Casaubon and Grandcourt respectively, and hasty. They don't marry for love. They expect to have freedom and power to do what they like, but once married, become absolutely powerless. Most importantly, they make the decisions themselves, being forced by nobody, and cause their own tragedies. However, there's no other way. Dorothea lacks experience and insight to judge people but, being intelligent and independent, often has her ways and doesn't listen to others' objections and advice. Gwendolen, selfish, weak-willed and used to comfort due to having been brought up a spoilt child, ignores everything and chooses the easy way to avoid hardships. Besides, Casaubon and Grandcourt turn out to be contrary to expectations only after the wedding. There's no other way for Dorothea and Gwendolen to learn. They have to make those wrong decisions and suffer the consequences, in order to become stronger, and wiser, and in Gwendolen's case, better, less selfish.
Reading Middlemarch may not be the same ecstatic experience as reading, say, Anna Karenina, but it's no longer frustrating once the readers accept George Eliot's method and get into the flow. Reading Middlemarch is like having the company of an insightful, wise, benevolent person who sees the foibles, weaknesses and mistakes of mankind and forgives them all, empathises with them all. She shows that wrong decisions can be damaging but not completely destructive, because we can learn from experience and become better.
2 lessons from George Eliot:
"Men outlive their love, but they don't outlive the consequences of their recklessness."
"If youth is the season of hope, it is often so only in the sense that our elders are hopeful about us; for no age is so apt as youth to think its emotions, partings, and resolves are the last of their kind. Each crisis seems final, simply because it is new. We are told that the oldest inhabitants in Peru do not cease to be agitated by the earthquakes, but they probably see beyond each shock, and reflect that there are plenty more to come."