How can I get my dog to behave on walks?

She's a pretty big dog, and I'm a pretty small person lol. She pulls like crazy, and if a squirrel or something shows up I can hardly hang on to the leash. I've tried jerking the leash when she misbehaves and stopping when she gets too bad, but nothing seems to work. If I stop and she wants to move, she'll just keep pulling and choking herself to the point where she's wheezing! I'd like to be able to walk her- she's overweight and needs exercise- but I can hardly control her. Choke collars are out of the question, and I don't think they'd work on her anyway. Any advice?