i don't know what to do here

i love my boyfriend with all my heart, and i want to marry him one day. he's absolutely wonderful (i was gonna say perfect but he's  not perfect but i love him anyway because he's my kind of perfect). he makes me laugh every single day, he's made me cry like four or five times but two of those were tears of joy and happiness, he makes me so happy, when i'm with him i feel content, he brings a light into my life with his smile, his eyes are a beautiful blue and they sparkle when he's having a good time or when he's laughing, and i love him with my whole heart, mind, body, and soul. 

but i've had thoughts just pop up into my head lately, like i'll see another boy and think he's cute and then i'll feel guilty for two days. i'll imagine what my life would be like if we hadn't met, and i'm nauseous for a five days. i'll picture how my life would be if i hadn't broken up with my last boyfriend, and i'll feel even more guilty and anxious for a week. i'll imagine what sex would be like with my own boyfriend because we haven't had sex yet, but, very rarely, does this happen, but i'll imagine what sex with someone else would be like, and then i'll sit here with this guilty pit in my stomach for like a week n a half. there will always be the scenario in my head that he might cheat on me with one of his best friends, like the underlying worry is always there. they'll just be hanging out alone one time and it'll happen and i'll walk in and bam world shattered. he claims she's like his little sister but damn i feel like they have a better connection than him an i do. and then i feel guilty and horrible an like a terrible human being because i know neither one of them would do that. plus she's also one of my best friends. and i just can't handle this feeling anymore and i don't know what to do. even after just writing this out i'm  so nauseous. just thinking about it makes me wanna throw up. 

is there something wrong with me?

is this normal?

breaking up with him is the last thing i want to do. he makes me a much better person. i couldn't live my life without him.