T. T,
You don't say what kind of "bull terrier" you have, and this could matter a good deal. If it is a pit bull, you may have some special problems. If it is a Boston Bull, things may be simpler. First, for Heaven's sweet Sake, lose that electric collar. All it does is hurt your dog who is already clearly ashamed and worried about her behavior. Believe me, it won't accomplish anything good.
Second,your dog sounds like a conscientious guard of you and your home. At seven she may be beginning to have hearing problems, which is one of the reasons I asked about the breed. If she is beginning to doubt her hearing, or if someone approaches your door and she misses sounds she has prided herself on detecting before the bell rings, she will be very likely to bark furiously to cover her embarrassment, just like a human, and to be very sure you are properly warned and protected.
What she is doing, for whatever reason if not this common one, is trying earnestly to take care of you and do her job, so you can see it is no time to punish her. Neither, of course, do you want to encourage her to make a racket or to be aggressive.
If she is a pit bull of any kind and is physically aggressive, you have an entirely different and sadder problem. These poor dogs, who tend to be loving, beautifully faithful family pets, are deliberately bred with a fighting tendency which is unpredictable and can be deadly. The monsters are the breeders, not the dogs, who are doing the best they can, but a pit bull who has become aggressive is an imminent danger to you, your children, and the community at large.
No, it is never too late for training, and you can definitely teach an old dog new tricks. With a dog you have something like 4-7 seconds to associate behavior with response, which means that if you respond to her barking appropriately within that period she will get the point which she would otherwise inevitably miss. So when she barks aggressively, if you can somehow gallop over, put one arm around her shoulder and one hand on her mouth and shush her very gently and quietly in a way to communicate the sense of quietude as a desirable thing, she will soon know what you want from her. Many repetitions will get across that she is supposed to do it every time. A possibly more practical, if less consoling, effect can be achieved with a short, clear order ("Hush!") said immediately but without punishment every time and a "Good girl!" on those occasions when she gets it right, this last especially. If you can enlist your children in the effort, perhaps competitively to meet the time limit, you might see an encouraging change quite soon. I suggest you ask your vet about her hearing and, if you don't already know, find out her breed. A single, self employed/student mom with three human children may well have trouble finding an affordable vet, but PAWS, the Humane Society or any other local animal support group might be able to refer you to someone helpful, including a good trainer. Do not, repeat, do NOT apply to any trainer who says you have to be tough with your dog. It is not true and will lead only to grief all around.
As for me, I am a crabby old writer and wildlife rehabilitator who has raised and trained dogs, mostly wolfhounds, and other critters for 45 years. I have no human children of my own but have taught and enjoyed a lot of other people's.