Sometimes, things we think we have dealt with in the past, or things that we thought didn't affect us much, become a bigger deal later on.
It does seem like a good idea to talk about it with your therapist, you could introduce the topic by saying you're tired becuase you haven't been sleeping well, and then say why.
As far as sleeping goes, there is no reason why you will relive the experience when you go to sleep, but convincing yourself that this will be the case makes it more likely. Instead, create a bedtime routine, full of things that make you feel safe. Wear clothes/pyjamas to bed, have a warm drink, cleanse, tone, moisturise (or not), read a famiiar book/watch a familiar tv show, and then go to bed.
If you have nightmares, wake up, get up, walk about for a few minutes, remind yourself of where you are, what time it is, that you are safe now, and that what happened was in the past. Make sure you actually wake up properly, as otherwise you can fall back into the same phase of sleep and dream again.
You can also leave a light on, which I find helps some times.
You're not alone in reliving things that you thought you'd dealt with. The key tends to be communication. Keep talking and sharing, shine light on the demons and they fade away.