Relationship Advice

Okay this is kind of a lot but I need some advice. I recently started taking birth control and I’ve noticed it’s changed my mental state quite drastically. I overthink and have been having some more concerning thoughts that have been putting my relationship with my boyfriend to the test. I’ve never noticed this about myself before, but recently i have been finding other guys attractive and it makes me feel horrible. I overthink and question myself extensively, and before I thought it could get any worse, I started picturing other people during sex. I talked to my boyfriend about it and he has been very understanding but I know it hurts his feelings a lot, and I want to stop these thoughts as soon as possible. I feel guilty every second of the day and it’s hard to sleep at night. I find myself not wanting to have sex at times because I’m scared I’ll start imagining other people, and it really is putting a drift between my partner and I. I do not enjoy these thoughts. I feel sick to my stomach and cry about this issue almost daily. The problem has worsened to the point where I now have thoughts about his family members (brother cousin etc). He said this happened to him when he went through puberty but I have never experienced this personally and what a worse time than when you’re in your first intimate relationship. (He is my first sexual partner, we are both 19). I am so scared that I am going to ruin what feels like the best relationship I could be in and I would be so heartbroken if that happened. Please please help.