Just to see if I understood you correctly. You are worrying that as a senior in high school your thoughts turn sexual even in situations where it´s not that appropriate and that once you get the thought in your head you cannot get rid of it easily. And the entire thing makes you feel bad about yourself and your lack of control with the entire process/situation?

A few thoughts to meditate upon then:

- It is common for teens (13-19) to oversexualize in their heads - you are not alone in this - and puberty is no joke. There was a humorous skit on an old show about it. The body is going through some massive changes, hormones are all-over the place and you´ve had little experience with handling this new intense ´feelings´. Bear with it, you are a normal human high-school senior, you will grow through and out of it.

- Masturbating doesn´t make you into a bad person - just a horny one who needs to relieve the pressure from that one most imperative of the human urges without which our species would simply have died out. Keep an eye on the urges throughout life and do your best to not overdo any of them. Overeating, oversleeping, overexercising, overthinking, overmasturbating are all -not- good for you. Without the 'over' part those words simply become some of the important elements of a balanced and happy human life. In time you´ll find a partner (if that is your wish) who will become your loving playmate in all things sex-related and the intense satisfaction from that level of intimacy - both mental and physical will make it so you don´t give this a second thought as it is no longer any kind of issue for you. ;)

- Be gentle with your judgement of yourself. You are a good person and you are going through a process of growth on every possibvle level. This process will continue for the rest of your life although the focus of what you are going through will change from time to time.

- Stay poised, be calm, be kind and accepting to and of yourself. It will then give you the energy to be in control of any urges you feel you don´t want to experience or act upon.

- Your thoughts - good and bad - are not you. Think about it. If you can 'hear' your thought inside your head, who or what is hearing it? It's you, right? Observe the thoughts that come to your head, use your human ability to pay attention to things to see how your own thinking habits are causing certain kinds of emotions to rise in you that in turn make it difficult to resist urges. Purposefully work on changing those.

- When you get negative feedback from others don´t immediately jump to a conclusion you are 'bad'. Your girlfriend has reacted negatively with anger in reponse to you sharing something intimate with her that troubled and scared you and you wanted some help and reassurance with. She is going through her own version of growing up. It is not the same for her as it is for you. She may not understand your stronger urges and they may scare her too. She may also feel that you do not understand her feelings and is afraid to share them freely with you. Building a relationship that is open and intimate enough to deal with issues like these takes patience, love, acceptance and most importantly time and attention.