Sorry that you’re feeling this way, it does sound very confusing and unsettling.
I would say - the grass is not always greener but it might be. You haven’t given any specifics so I’m assuming that’s because you haven’t figured out what aspects of your life seem to be unfulfilled.
If I were you I would work on that - what is it that you are actually missing. You can do this while with your partner (or single or with a different partner) so there’s no need rushing to break up before you actually know what you want. You might find that breaking up doesn’t magically fix the problem, as you never really knew what the problem was to begin with.
If your partner is supportive, you should be able to share your feelings with him. (I probably would leave out the part about other men or ‘is this relationship working’, and instead just focus on how you have been feeling down sometimes, perhaps want more from life but not sure what). He might jump at the chance to find new ways to explore life with you, or he might give you space and understanding to do what you need to do for yourself.
You might also find therapy a good place to start with all of this. I’ve tried private and nhs support in the past. 1-2-1, face to face sessions are worth the cost if you can afford it.
Once you have a clearer understanding of what you think is missing, you might decide to move on from your relationship. If that’s the case, I think you’d feel more empowered, more sure of your decision and happier to do so as you will have a clear understanding of why you’re doing it and what you’ll do next. I also think it may be less likely for you to get back together with him out of fear and/ or end up in a new long-term rebound relationship.
Good luck