So what actually happened to susan in the end?

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In the book, she sort of returns to Herbert - after months of troubled passion Tom and Sue realize they can't really start anew together, it would shirk the relations both with thir children and with their respective partners. As pointed out by some others here on the board, Susan's style as a psychotherapist is strange: it's rule number one for any psychoanalyst or therapist that you never, NEVER, get into a sexual bond with the person who confides in you, that's just out, not just because of the Code of Honour of Medics but also because it distorts the way the headshrink could act as a talking partner, a symbolic other to the patient. Tom isn't even her patient proper, he just arrived in Manhattan to answer a few questions but as the story goes on, he becomes the center of things to the exclusion of his suicidal sister. Anyway, no psychologist professional would get in bed on the therapy sofa.

The book has some great stuff in it - the style and storytelling when it deals with the South is very inspiring and fun - but it's also got something fundamentally flawed and bogged down in its treatment of Savannah's disease, Tom and the psychiatrist. I think Conroy is by and large just using the psychosis stuff as a McGuffin to get the book moving: we never really get what would be so essential in the stories Tom tells Susan (though they are dramatic) when it comes to Savannah's psychotic behaviour. That impossible rape-and-robbery story at the end is the worst of all - I do feel he's just using it as a grand climax; it doesn't explain a thing, and Mrs Wingo's decision that nobody must ever know and that she'll pretend it never happened, that not even her husband will know, doesn't save it. I get the impression that Savannnah has treated all of these things in her poetry before, that they've ben the very growing-bed for her writing, but that doesn't seem to have helped her, it just pushed her to flee her own identity (the Renata Halpern bit). This makes the final plea of Tom to his sister "Let the whole world love Luke Wingo" completely hollow (she'd already written about her brother), but yes it jerks tears outa people.
The book, and its finale in particular, sort of buys into a cheap Alice Milleresque trauma theory which in the end would just chain down Savannah to her malaise, but then the real subject isn't Savannah but Tom's romance with the doctor.

in the book, she ends up dating a lawyer and writes to tom and sends her love.

First of all,you are talking about the movie not the book,so for get about the book.In the movie,it is appiled that Susan goes back to her husband,but I think that me and my best firend are going to rewrite the movie and have Tom and Susan together and Sally and Jack together and Herbert and the panio player together.I am not an expert on the book,however I did see the movie in theather ten times,I have been to Beaufort where part of the movie was filmed. I have the movie on tape and I have several poster from the movie.Besides,I never felt sorry for Susuan.She what she was getting into when she started having an affair with Tom.Sally was right when she said that she can't walk away from alife if she can't make it work.Anywho,Susan knew there was a chance that Tom would go back to his wife,that's what she gets for havign an affair with him.