moving on from your first love

Okay this issue has been bugging me for way too long and I just need to let out what's going on inside my head. So I have a boyfriend of 4 months and he is the sweetest guy although a lot skinner than what I usually go for and definitely a lot more sensitive (he cries more than I do!) but i can't stop thinking about my ex, me and him dated for about 10 months and he was definitely my first love I ended the relationship feeling as though it wasn't going stay forever. I just can't seem to move on fully and it's effecting my current relationship cause I really want it to work out but I can't stop thinking about my ex