Think of other women constantly especially while having sex with my girlfriend of 4 years

Hey guys I'm just seeking some relationship advice/information from people who have been in the same place as me and what did they do. I have been with my girlfriend for 4 years. I am 24 she is 23. We live in different cities but see each other every weekend at least. Plus we both have holidays and time off. Basically I think about being with other woman constantly (even while having sex) and crave the company of other woman. I flirt with plenty of women and feel like if given the perfect opportunity I would cheat on her (basically if none of my friends would know), i know this is awful. I have felt like this before but it has gone away. She wants to settle down and move in together I am nowhere near ready for this and feel I am dragging my feet in this relationship and feel I am much more irritable around her. I love her but I know I want to be single in the near future. If I broke it off with her it would be a meltdown for her shes going through a tough time and she is so sure that we will be together forever. If I feel like this now how will it ever get better but I am mega scared to end it. Feel like a whinger but figured fellow redditors may be able to relate to this and help me out.

TL;DR Disenchanted in relationship want to have sex with other women feel. bad for girlfriend.