I Can Empathise
I have a close friend who is in a similar dilemma. I love her dearly and could empathise with what you are going through. How about I share her story and why don't you contemplate it?
She is a beautiful, intelligent girl with a bright future. She has a great sense of humour and is wildly attractive. The only drawback is her parents. Just as your father abused you, they put her through psychological abuse through their constant arguing and manipulative games.
She Has No Faith in People
Since as children, our parents are the heralds of the world to be, we all create a mental blueprint of what to expect of the world from the way our parents treat us. So she grew up believing the world to be a cold place without much warmth. She does not have much faith in any person, considering the people who ought to have loved her ended up harming her; and so she never gives a chance to any guy who comes into her life. Somehow, deliberate or subconscious she ends up destroying a would be wonderful relationship. Is this you?
Fear of Getting Hurt
You mentioned something: that after much self-analysis you've come to the realisation that there is a dark side to everyone's sexual side. Be honest with me, are you saying that everyone has a nasty side to their sexuality? And are you afraid of getting hurt again? In short are you afraid that if you open up, that guy may end up hurting you like your father did?
In your letter, you did not mention that YOU loved Tim. Are you afraid of getting too close? And is your infidelitous side a means of achieving that?
Your letter shines through about a girl who is self aware, intelligent and sensitive. She wants to be loved and desperately wants a warm, lasting relationship, which she could look to the future to.
To me you cheat because you're afraid, rather tasting the variety of life in a hedonistic way. To change it involves confronting what you're afraid of, and to me it's the fear of getting hurt.
Be Honest with Tim
If you are serious about Tim, I'd advise you to open up to him and be completely honest with him (NOT about what's happened) but about your past and where you're coming from relationship-wise. Talk to him about your fears, just be your honest self and don't be afraid of saying silly things or getting hurt. Do this so that he knows how you perceive a relationship to eventually unfold and how you want to challenge that.
Cultivate a Strong Circle of Friends
I'd advise you, if you haven't done it already, to cultivate a strong circle of friends who know you completely, and care for your dreams and well being and are willing to help you achieve what you want.
Seek Counseling for Abuse
The shocks of abuse when experienced at a young age, ripple through the years into adulthood and affect us in many negative ways. To challenge it, there is only so much you could do through self-analysis and acceptance, I would sincerely advise you to seek counseling from people trained in helping those who've experienced abuse. And please, don't feel bad about seeking help, because it's a very positive thing to do (because you're doing it to make your life better).
Guilt Is Fultile
Finally, guilt is a very heavy and futile burden to carry. There is a saying which goes "Don't regret the mistakes you've made, only those you're about to make". The fact that you've stood up and admitted that you want to change is the biggest gift you've given yourself.
What do you think of this Answer?