It feels understandable to me (though horrible for you) that you have these vivid, disturbing images… your mind has identified what it believes is a threat, and feels the need to return to the subject again and again to make sure you're prepared and can keep yourself "safe". It's extra-vigilant and always ready to see potential occurrences of the thing, and uses any opportunity to model predictions and come up with scenarios, then produces vivid mental representations which it then can't help but focus on.
It's all so bloody unhelpful, though, and I feel for you. I know that taking meds for MH problems isn't the same as taking them for epilepsy — I mean, the very reason I take meds is to change how my mind functions — but FWIW I can totally empathise with that feeling that a drug is changing you in ways you don't want, but where the idea of coming off it and trying yet another unknown quantity is just as worrying.
It's possible IMO that the drug is predisposing your brain to this kind of thinking (I'd be suspicious of it in your place too, as lamotrigine does seem to be able to cause this kind of thing, and your timings do line up — but I'm not a doctor, or anything close to it). It's also true that might not be related to the drug. Even if it is the drug causing it, that might not necessarily mean you need to come off it — some of the treatments used for the problems you're having with these unwanted thoughts can probably still be used, medication doses can be adjusted, and so on.
But really, you need to discuss it with whoever decided you should take these drugs and/or whoever's prescribing them.
If you were getting worrying physical symptoms that were interfering with your life this much, and you suspected they were being caused by a medication, I would hope you'd have the opportunity, and feel able, to discuss it with a doctor who can help you move forward with this. So, if this is a side effect of the drugs, it deserves to be taken just as seriously as an equally-troublesome physical side effect. (And if it's not a side effect of the drugs, it equally deserves to be taken seriously as a health problem.)
Seeing the doctor who manages/prescribes your medications is really the only way to determine whether it not it's likely to be a side effect of the drugs, and what actions to take. I'd guess those actions could be drug and/or non-drug treatment for the worrying/intrusive thoughts, adjustments to your epilepsy meds, both, or something else, but I don't know much about epilepsy treatment except that they often seem reluctant to withdraw something that's even partly working.
I suppose it might be a bit tricky because the side effect is in a different field of medicine from the condition being treated, and the NHS can be difficult to navigate in these circumstances. But side effects outside of the strict remit of neurology can't be unusual in people taking medication for epilepsy, surely?
I know how when it's your own thoughts that feel bizarre and out of control, it's really upsetting and alarming, and you said about how embarrassed you feel, but I promise that these kinds of difficulties are not at all shocking to doctors. They will not think you're weird or disgusting or dangerous or crazy, or conversely that you're fussing about nothing. They'll recognise that you're experiencing a variety of mental glitch that happens to LOTS of people, where systems that are meant to help us can go a little awry (either on their own or pushed by a medication), and which they're trained to identify and offer treatments for.
It's shit that you've got to deal with this when you're still trying to get on top of your epilepsy, and presumably have all the other stuff going on that life generally inflicts on us. But the kind of problem you're describing isn't something that you should just have to accept and live with. I hope that it won't be too long before you're past all this experimenting to find a tolerable medication regime that works for your epilepsy… I know it's not always that simple, but I can only wish you good luck.
Sorry about the lengthy post(s)… it's a side effect of one of my medications 
(It actually is. I wish I was kidding.)