Love is a feeling, so when that feeling was gone, then I knew. Being with them felt like a chore. Years together sounded like a sentence rather than an opportunity. Feeling their emotional connection towards me would make me feel uncomfortable and awkward because I didn't return it. I felt neutral towards them and like the only thing keeping us in a relationship was inertia. Incompatibilities kept growing and becoming more and more obvious. We started living more as cordial-at-best roommates rather than partners. The feeling of love (or in this case more likely limerence confused for love due to youth and inexperience) had simply dissipated into nothing and there was nothing deeper to build on.
Treating love like a feeling is a very modern way of looking at love. Love is a commitment and a choice not a fleeting feeling as a feeling comes and goes. You wake up everyday and choose to love your partner. It's not like in the movies. Love takes work on both ends. Try to slowly open up to him again and build back what you guys had.
Instead of fantasizing of other women you start flirting. You never think of your partner while at work and you're not excited to get home to them. You dont like talking about them to co-workers and feel like a liar when you say everythings good. Dont like hanging out with your partners family/friends because it now feels kind of fake. You ponder what friends and family will think if you left them. You start to resent them. Have troubles in the bedroom because you aren't as attracted to them. You buy dinner, gifts etc but your hearts not in it.
If you find yourself completely unable to take them seriously and roll your eyes mentally at everything they do, it's a pretty strong sign that you're falling out of love. So you need to either assess whether you're being unfair and need to change your attitude, or if this is the new normal and it's time to end things.