You're not alone in that statement. Most people think the ending is a little rushed, myself included. It's a fair comment I think. However it's not really the point: Austen has told the story that interests her and all that remains is the summary.
I think the problem is peoples expectations. They expect some kind of romantic coming together and this book is not really a romance. It's about Sense (good sense, common sense) and Sensibility (emotionality, and lack of good sense) and Austen is not really interested in the romantic element in this book, the marriages at the end are a convention, a mechanism to finish the book. Austen wrote within a narrow convention permitted for women writers at the time and her novels have to finish with this convention whether she likes it or not. P&P could be described as romance, and Persuasion too, but this is not what interests her here. It's closer to tragedy: Marianne ought really to have died.
You're not alone in thinking that certainly most modern readers feel like that. Marianne is one of the great tragic figures in literature but really you've missed the point of the book if you think that Austen thinks her behaviour is in any way laudable, and personally my sympathies lie with the author's.