If you think you're going to take a serious medication that will alter your brain and have it be problem free and also solve this problem, probably not so much. It's great to have meds when we need them, but if this is your only problem, I wouldn't think about magic pills to solve it. I have no idea what thoughts you're talking about, or what age you are, or where you are in your sexual life, so it's hard to say what's going on, and you also don't say how long this has been going on. You also don't say why this would cause intolerable pain, as sexual thoughts are usually pleasant, so are these thoughts about unpleasant sexual experiences? Basically, if you're thinking things obsessively you don't want to be thinking about, you have to stop thinking them by either by relaxing so whatever stress is causing it eases up, figuring out why you're thinking these thoughts (such as, something happened to cause this, some guilt you are carrying around about something that happened, something you saw affected you so much this occurred, etc.), or just stop thinking about this and think about something else. Not so easy to do, but it is what will fix the problem. Meds won't fix the problem, even if they make you feel better temporarily. If this is truly this bad and you can't stop the obsessive thinking, therapy is the place to go so you can get help figuring it out and learn some relaxation exercises and cognitive behavioral exercises to help you move past this. Peace.
Intrusive thoughts are such a bear. My son suffers these as well. They could be from obsessive compulsive disorder such as with my son. Have you ever been diagnosed with that? I would suggest seeing a psychiatrist. My son has been undergoing treatment with an SSRI as well as psychotherapy using CBT and DBT and has had success in minimizing intrusive thoughts. He reports this himself. This comes as a welcome relief as his thoughts were such that he said his purpose in life was to die. gulp. So, quieting your mind is something I understand.
The human brain can only hold one thought at a time. So my son also has success with changing what is being said. He has some go to phrases or images or things that he can immerse his thoughts into that then drown out the intrusive thought. He also has learned that distraction is OKAY and helpful. He has some activities that when he engages in, he's thinking less. We encourage him to jump to those activities. And the more you do the pleasurable activities, there is improvement in how you feel.
Since your thoughts are of a sexual nature and they are bothering you, I do wonder if talking through things would be of benefit. Don't answer here, but where do these thoughts stem from?
For dangerous intrusive thoughts, some doctors will prescribe a low dose of lithium. You can read about this. This has a high success rate. We didn't go that route as the intrusive thoughts are a symptom of the mental disorder for my son so we are fully treating the mental disorder. My son takes an SSRI with success. All parameters of his mental health are improving. He also is engaged in the different types of talk therapy he is doing and he does things related to that on his own at home (has work books and such).
Night time can be particularly hard for intrusive thoughts. My son did use a medication given to help him sleep temporarily. That's always an option. But again, his intrusive thoughts were dangerous and of self harm. So, it was necessary to quiet them for his own safety at night. He no longer takes the medication that made him sleep. But sleeps and is able to control the thoughts since we are treating the depression and anxiety and ocd.
I suffer from intrusive thoughts too. Lets just say they aren't violent or physically hurtful. That said I try not to think of them by counting various things in different categtories. I do this in bed at night. So far I've remembered 142 SNL hosts.