How do I stop my dogs newfound aggressive behavior?

I have a spayed chocolate lab who is almost 9 years old. She had been raised with multiple dogs for the first 6 years of her life, and got along very well with no problems. For the last 3 years, she has been my only dog. Over the last 3 years, I have noticed an escalation in certain behaviors with her since she became my only dog. She shows leash aggression towards other dogs, and will fight with them if they get close to her while she's on a leash. If I am not in the house with her, she will bark for hours and does not stop (I tried the bark collar-she got used to it and ignores it now). She gets too excitable when playing with other dogs, and it usually escalates into a fight, initiated by her. My in-laws got a puppy, and she has snapped at the puppy twice while eating (food aggression). She has become a very nervous, bad-mannered dog in the car, and can't go for rides with me because she tries to jump out the windows and barks at everything she sees, especially in town. These are pretty drastic changes, and she didn't exhibit any of this until the last couple years. I have had people tell me to get her a playmate, but I am unable to have another pet where I live now. I've read all the dog whisperer books and tried the techniques, but I'm failing miserably and don't know what to do. I don't want to give her up. I really would appreciate some sound advice on this situation. I live with my boyfriend, and he's about at wit's end with her.