I've been with my partner off and on for 6 years now. During one of the off periods I did have a fling with someone else and so did he but lately I am finding myself fantasizing about other people, the worst is that it is someone from work and not just a random guy. I just can't get it out of my head.
Our sex life has taken a huge dive lately as he now works nights and I work very long hours so we live on opposite sides of the clock which is hard enough in itself. But I also feel really bad as the idea of this other person gets me off but when we do get a chance to have sex I'm unable to get excited.
I've always felt like monogamy isn't always suitable and is just the convention of society these days and have thought about discussing an open relationship with my partner but really don't think any such discussion is a good idea when my judgement is clouded by these fantasies.
Any advice? Could something like this even be stress related due to work pressures etc.

Fantasies often get stronger when partners are apart for whatever reasons. That's normal, as we can't always control feeling that trigger fantasies or dreams. No biggie until you cross the lines of good behavior to act on those fantasies. No matter how intense or realistic they seem to you, fantasies are best kept to yourself in fantasy land.

Just wanted to chime in. Open relationships and polyamoury are things that take just as much if not more work then conventional relationships. They seem nice and wonderful and they can be but a lot of human emotion gets invested and it can go REALLY bad if it isn't requited. Jealously, neglect, and other emotions will cause hurt feelings and damage relationships, even if everyone is on the same page and agreeing.

That being said it is something you can explore, just make sure everything is transparent and ALL your partners know what is going on.

Anyone who claims they have never had a fantasy about someone other than their partner either is extremely young... unhealtily obsessed... hasn't been with them very long.. or just flat out telling a lie.

Fantasies are normal... its acting out on them is where people can get into trouble sometimes.