I only have limited information but I believe that you should be concerned. One major red flag is that he was following you. You believe that it was only a one-time occurrence but you don’t know that for sure. There could have been other occasions that you are unaware of. His behavior could be considered stalking.
Another red flag is that he continues to feel very strongly about the breakup. As you noted in your letter, you were able to move on with your life. He has not been able to do the same. This is not a normal reaction to a breakup. Breakups are upsetting but over time most people come to accept the fact that the relationship is over. They move on with their lives. The fact that considerable time has passed and he has not accepted reality is a concern.
Another red flag is that he has a history of abusing you. You did not detail the full nature of the abuse but the fact the you were forced to move into a shelter is a sign that it was significant. Psychologically healthy people do not abuse their partners. This is yet another example of his psychological instability.
The fact that nothing has happened yet should not be taken as a sign that nothing will happen. Some of the most violent attacks occur without warning. Research shows that individuals who are separated or divorced are at the greatest risk for stalking. In addition, women are more likely to be victims of stalking than men.
My advice is to be on high alert. Take precautionary measures in every situation. Do not hesitate to call the police or file a police report if something suspicious occurs. Do not minimize any aspect of his behavior. Stalking is about power and control and in that way it is very similar to domestic violence.
If he attempts any additional contact with you, file a police report and inquire about an order of protection. I would also encourage you to read about stalking and how you can best protect yourself. Every state has anti-stalking laws. Know the laws in your state. Two helpful resources are the National Center for Victims of Violent Crime website and the National Institute of Justice. I hope this helps. Please take care and stay safe.