I find myself lusting for ''other guys'' when ive had the same boyfriend for 4yrs now?

Whats wrong with me? I have been with my boyfriend for lyk .... 4yrs now & hes the first guy ive ever had with & went the furthest with but for a qiet few months now i have been so thinking of ''other guys'' i picture myself with them doing all this ual stuff & i cant get it outta my head i feel im bored with my boyfriend & lost interest in him & just want new things? I tried telling him how i feel & he just says iff i ever touch another guy he will cut that guys d!ck off (LOL) so what do i do i feel trapped i just want to move on my ual lust just isnt their for him no more i feel bad & sorry for him though. Is this wrong? How often does this happen in relationships? It also makes it harder cause i have a 2yr old child to him i love my child so much i just need answeres me & my boyfriend are only 20? I just feel i want other guys BADLY