Rarely does an office romance lead to a productive end, as you're now experiencing. Many companies are trying to establish policies about this issue, but it isn't reasonable to expect that any policy will keep it from happening entirely. Whenever human beings come in close contact and share time and energy, some will become involved romantically. The only advice we can offer on the personal front is to learn from this experience so you don't repeat it. As a new hire, it's almost guaranteed that if push comes to shove, you'll be the one to lose, not a senior executive.
On the career front, you have a couple of choices. You can quit immediately to remove yourself from a situation that isn't likely to improve dramatically in the near future. Even if you believe the market isn't favorable now for job hunting, with diligence and persistence you'll find another job. You found this one a short while ago, so you can once again apply the successful techniques you used earlier to find another job now.
If you think you can weather this storm and the company is large enough, ask to be transferred to another division. This way you can still contribute to the company, but won't be reminded daily of the relationship with your former boss.
If that isn't possible and you're a good actress, you can carry on as if nothing happened and concentrate on your work to the exclusion of all other factors. We think it's unlikely that you'll carry this off since you already describe the situation as unbearable. However, it's one strategy that others have used to overcome unpleasant outcomes from office romances.
Whatever you choose, be realistic about your limits to endure this situation. Use better judgment from here on so that you never again place yourself in the position of risking more than you're willing to lose personally or professionally.