GoFigure12,
Coming in late here, but again, welcome to Male Survivor and I hope you will stay with us. Just find your own pace and post on any issue or question you need to talk about. That's what we do here.
I can well imagine how this one is tearing you up:
What am I supposed to think when in my masturbation fantasies I sometimes go right back to the times he abused me? I can fantasize about him raping me, so many years later, and reach orgasm over the fantasy.
This of course goes right back to the fact that you did experience orgasms with the older boy and saw the whole relationship in at least a partially positive light.
For one thing, every boy wants and needs validation, affection and approval, and wow, to have an older boy show interest in you? That must have seemed, as you say, pretty cool. You were too young to understand that he wasn't interested in you as YOU - he was a predator. This oversight was NOT your fault.
And orgasms? What were you to understand from that? I remember when I was first being abused at the age of 10 and had my first orgasm with the abuser, I was amazed at the great feeling, even though I was also scared, embarrassed and very confused. So what did I do? I asked the abuser! I asked him because it feels good does that make all this okay? His answer was of course yes, yes, yes. What does this show? It shows that I was too young and innocent to understand what was happening or to recognize that I was being used. At 13, the same would have been true for you.
As Kenf has already said, the orgasms just show that you were (and are) a sexual being. We all are. You will find so many guys here who experienced the same thing and felt the same way about it until they discovered how common this is. A boy's body will so often respond to stimulation regardless of whether he wants or likes what is being done to him. This is just one example of what we can learn in a place where we can talk together with other survivors.
On your sexual fantasies now, it's useful to remember that our feelings don't always reflect the way things really are. You are in the first stages of recognizing yourself as a victim of abuse and understanding what was done to you. The old feelings you had as a boy remain pretty powerful, and when you fantasize as you masturbate you go for what's familiar: the sexual memories you have as a boy looking up to an older kid who paid attention to him. There's nothing to be ashamed about in that.
There are two particular themes I hope you will take away from my response to you here. First, NONE of the abuse was your fault. You may not see that right away, but it really is a truth that will become clearer and clearer to you over time. The second is this matter of feelings. Abuse teaches a boy a host of false lessons that fill him with guilt, shame, and a conviction of his worthlessness. But you did nothing wrong; the blame lies entirely with the abuser. You have nothing to be ashamed about; you didn't have real options and could not have understood what was happening to you. And finally, your role as husband and father, your worth as a man and the importance of your life are not touched in any way by what an abuser did to you.
This is a lot to take on board, I know, but it's all true, really!

Hang with us and see for yourself.
Much love,
Larry