What do I do 😔

Okay this is so bad and I feel horrible for feeling like this !! But I can't help it I cry for hours over this Im just so unhappy, 
Okay so I've been with my partner for 4 years and we have a 1 year old and trying for number 2. (Which doesn't feel right Atm but how to tell him that) 
But I just don't feel i the same towards him anymore,I'm just not happy 
He introduced me to one of his mates around last year and
I just can't stop thinking about him they don't hang out a lot but when they do im allways there and he is just amazing Im absolutely head over heals for him. Seriously what's wrong with me I barely even know him, I dream about him I can't get him off my mind and it's annoying, I'll be having sex with my partner and I find myself wishing it was him.  
I'm a horrible person I hate myself for not loving my partner anymore
What do I do 😔 only kind words please