What do you do when your husband knows your not happy but wont let you go?

Ive been married for 18yrs and believe me none of its been a happy memory,Three kids later,a house,I find we have nothing in common,He basically gets home from work icnores me glues himself to the tv and passes out,its the same routine day in and out,to the point im bord to death...Ive tried to leave many times over the years but he makes me feel guilty and begs me to stay!For every year that passes i feel more and more empty i dont even think i have any love left for him anymore,I wish i could walk away so maybe one day i could meet someone who actually cares about what makes me tick someone who wants and has the same intrest as me....I just wanna run,but dont know how,,,