Well this one thing about istikhara; my family always discouraged us to take it. Their take is that you have to be extremely strong willed b/c if the results are not in your favor, you have to follow them anyways.
Also, Allah has given us common sense and He puts us into situations where we have to choose b/w A and B. Half the Pakistani Muslims do istikhara before getting married. But Allah shapes everyone's destiny and tests us in ways that we can't understand. Despite all the istikharas, people die, marriages don't work out, tragedies happen.
Istikhara should really be initiated when you have 2 options that you like and you can't make up your mind on which to pursue and hence you ask for Allah's consult. But if your heart is already not set up on something, what would you take istikhara for?
However, prayers can change anyone's destiny and Allah is always listening to what we're saying. Best thing to do is to pray for yourself, your future and happiness in your life.
My uncle and aunt were VERY happily married for 2 years after which my aunt ran off with a Hindu guy, leaving a daughter and a 2 weeks old son behind.
Stop worrying for yourself like this. You can't know how happy or sad you will be with the guy you like if you guys were to get married. And similarly, you also don't know that you might just meet the love of your life thru an arranged situation.
As for forgetting anyone, happiness comes from within. If Allah thinks that this guy is not deserving of you, and you of him, He wont make it happen. And then it's upon you to find happiness in what He's given you, and trust me, that is always a part of the package. If your husband is a nice, loving guy, there is NO way possible that you will cling to old memories. And again, you never know what beholds your destiny. Maybe you will somehow end up with the guy you love, or you think you love since you haven't experienced anything better.
And pLEASE stop worrying about things that haven't even happened yet. My prayers are with you!!