It occurs to me that while you both said some dreadful things in the heat of the moment, coming from hurt pride or emotional pain, that you didn't mean - the end result of this break-up (and even his taking offense in the first place) has more to do with OTHER issues around your relationship than just this business venture. If you (or he) has another relationship, situation or issue which is unresolved and hanging in the balance... this could have been a "safety-mechanism" for him to pin these feelings of insecurity or confusion about whether to continue your relationship on the career advancement issue when in reality there is a lot going on.
My advice to you would be that you continue to not make contact. If he felt the same way about you (or at the very least was ready to "do" something with those deep feelings) he would move past the things said in the heat of passion... because he is not, and because in his letter he's being conciliatory but very obviously "distant" (I wish you and the girls the best in your lives) you have no recourse but to respect his take on the relationship and stay back as he processes things.
If there's something you're not willing to see about this situation -- now is a good time to take stock. You may have fallen deeply in love but are you telling yourself you can't get this feeling from others/your present life?
Wishing you all the best.