i would say fear of being alone

letting go of something before you have grip on something new can be scary, esp if you know that the “new one” is just not girlfriend material, but you are selfish enough just to get your dick wet

but i would say, if it’s too scary don’t jeopardize your existing relationship

i would know, i left my girlfriend for my now wife of 12 years - but i was honest, confessed that i fell in love with another woman and moved out the same day to keep it as painless as possible for her, didnt want to drag it out, and didn’t want to play her, she meant something for me once. Still felt sorry for her, but i couldn’t lessen the blow anymore than that :-(