In a long term relationship is it normal to experience buyers remorse and wonder if you could have done better?

I feel like im having a cold feet period in my long term relationship. We have plans to spend the rest of our lives together but i've been having unsettling thoughts lately. I'm starting to notice more attractive guys and lately i've been feeling like I could have done better. he's even said that he isn't good enough for me. I do love my boyfriend, he's my best friend and were great together but the little things are starting to get to me. I have a college degree and he never got his HS diploma so I worry about our future security. He's not as romantic as he used to be and I notice every little imperfection now. Is it normal to have these feelings? I couldn't imagine my life without him but I hate wondering what might have been, has anyone else went through a rough patch like this and what can I say to myself to put my mind at ease?