How do you get past seeing your ex with someone else without totally losing it?

My ex fiance who I lived with for 7 yrs. broke it off with me without a clue as to why he did- still no closure. Recently, friends of mine told me they had heard that he was seeing someone else. This relationship just came about 2 wks. ago. He may of been checking her out prior to that- something I will probably never know. When I questioned him after the breakup & even again last week if he was seeing anyone he said no. The girl he is seeing lives down the road from me so I knew the day would come that I would end up running into him & her together. Well, today is that day & let me tell you, hearing it & seeing it are two different things. I was on my way to work & lets just say I was functioning in a fog- like things were not real. I basically, shut right down. Of course, not knowing anything about her, all I keep on thinking is that they are going to go off & have this wonderful life together, a life we were suppose to have. He had been telling me he loved me very much right up to & after the breakup but, he just needed to be by himself to sort things out so as much as it devastated me I chose to give him the space he wanted in hopes that we could turn this thing around. First of all, how can someone spend 7 yrs. with someone, plan on getting married soon, & continue to tell you that they love you & then the next moment be telling this other person they love them very much & want to marry them. These words I saw written on a pad with a vase of flowers to her & were on the table at his place when I stopped by to pick up some of my belongings. When I read what he had written about loving her very much I felt like throwing up right there. Can he really " love her very much" in such a short period of time? It kills me to think that she is laying next to him in bed where I was just 1 mth. ago & have been for 7 yrs. Do you think they have a chance of surviving? Why couldn't he be honest with me when I asked him- he had to of known that I would see them together at some point soon since she lives so close by. Another thing I found out was that she has a dog & a cat which she takes to his place when she is out there & these animals are in the house but, when I had told my ex I would like to get a dog his response was that if I did it would have to be outside because he doesn't like animals in the house. Now she is there with a cat & a dog inside the house! This has to be just a nightmare I am having! Does anyone out there have any advise as to how to cope with all these feelings I am having so that I can function with my everyday life? I would also, like to add that my ex was a daily drinker, he liked his beer & I was only an occasional social drinker so this created conflicts now & then. He was never abusive to me & never drank during work hrs. or was unable to function. I'm not sure if this girl is a drinker or not but, that would probably make my ex happy if she was because he always had a problem with the fact that I would not sit & drink with him. Do you think the day will ever come that he will feel regret for what he has done to me? We were making plans for a life together & now he is off enjoying his with someone else & I'm left in dirt. I am normally a very outgoing confident person & now I am feeling worthless & lower than low! My friends are telling me to forget the loser & go on with my life as if it were nothing but, they just don't understand because there not feeling what I'm feelng! Please, help!