Protecting yourself from pain is to protect yourself from happiness.
I dont think you could go slow with ex. Cause once you get in contact you already investing emotionaly. You just have to decide if you want to take that risk again. She dumped you so you have to decide uf you want to be reused again. I mean maybe its better to keep honour. But the thing is that reason why you broke up is fixed now so you could be sucsesfull couple. Only risk is that you might go back to your old habbits with ex and all the work done will be gone. Its harder to change for good with an ex than with new girl. If she wasnt with you at your worst, she dont deserve you at your best. You had strenght to pull yourself up and start live better on your own. Will you be as strong together with your ex? Maybe she did you a favor by leaving you and you can find better girl.
Doubt kills more dreams than failure ever will