OCD thoughts hopelessness help please

Hello, I am struggling with OCD on the compulsive side. I have an issue lately that is horrible and I hate talking about these things from the fear of judgement. I have thoughts of bad things obviously but an example of what I am feeling I will write. 

So on Thursday I went and worked helping my friend at his greenhouse. It was him me and his mom and his little step sister. I saw his little step sister who is I think in the eighth grade and I am a senior in high school. My mind instantly sexualizes things most of the time but I did not in this situation. I was like yeah whatever they do not look bad and are not ugly or anything and tried to shake the thought. I sent the rest of the day and Friday not telling anyone how I felt and how I saw that situation.

Tonight I was talking to my girlfriend and told her about it and as you would have guessed she was not happy. I told her I would not act in these thoughts and I do not enjoy having them and I do not feel like that is me. Out of anger my girlfriend hung up and I messaged her about it but that is not what I am worried about. 

What I am worried about is that after talking about that and stuff I got the thought to go masturbate. I went and looked at some drawn porn (hentai) and then went to normal porn and back to hentai. I watched the video and got off to it and I can not help but to think that me talking about my friends step sister might have gave me the thought to go do things to myself and I feel guilty and shaky and I am not happy about it. I do not want tot elk my girlfriend about it in fear she will be upset and leave or anything. I do not like feeling this way and it makes me feel bad. 

I do not know if that is what gave me the thought to do it or if I just thought to go do it but I am thinking it was the thought or something of his little step sister and I feel bad and guilty. I do not want to feel like this and do not think I would ever act on anything like that. I do not know what to do and I feel like I may have thought bad and done something bad by thinking that way. Please help me.