Like Akirio said above, these insecurities are very normal in a relationship, even in a het one ("Omg my darling is so outta my league it's almost a guarantee he/she will leave me"). This is a happy oneshot because Shuuko at the end showed that she's a fighter who is willing to keep her girl even with those insecurities haunting her. I respect people like that a lot, because there's a balance between thoughtfulness and determination. Much better than the pathetic doormat who back off because they aren't strong enough to weather a hardship, and then give a pretentious "I'm only thinking for your future!" to excuse their cowardice.
Whether your partner should be with you or not is her choice, not yours. Your duty is to live sincerely, don't be a dick if she choose to walk off, but don't be a matyr if she wants to stay either.