Should I still be in love with my ex even though I am in a relationship?
How do you know you are still in love with your ex?
When I think about my boyfriend, I'm reminded of my feelings for my ex.
I can understand having mixed feelings, but you shouldn't get involved with your ex. They are your ex for a reason, something didn't work out. I wouldn't cheat on your partner to go back down a road where it only ended up in heartache.
What if I'm just stressed about my relationship with my boyfriend?
I think that is exactly it, and you shouldn't be thinking about your ex to make things better. Instead, you should speak with your boyfriend about your problems as that's a way better solution than potentially cheating.