Why am I jealous of my girlfriend's kids?

OMG I am the same way…I am 21 and my boyfriend is 29 and he has an 8 yr old little boy, and we have been together for 4 years. At first we didn’t have custody so it wasn’t bad having him every other weekend, but then we took custody for many reasons and ever since then I feel like him and I compete for his dad’s attention. He is smart and I feel like he knows what he is doing, especially when I go to sit on his lap and he jumps in his lap and smiles like haha. I know I sound like such a brat but it’s just hard. Don’t get me wrong I love the child as if he was my own and I treat him better than his mother would ever be able to and I do h.w with him and take him places buy him things. I think he doesn’t like me sometimes because I am really into being structured and having rules and he’s not used to that and he doesn’t like being told no and all that so he will go ask his dad something I already told him no about, thankfully though my boyfriend understands and once I brought it up he paid more attention and told him he has to listen to me just like he does daddy and that he needs to respect me at all times even if he doesn’t like what I ask him to do etc.. I think I have gotten more used to it over the last two years, but I do feel deep down a jealousy of him. Like when my boyfriend is watching tv in the living room he will come back to the back room to sit with me and as soon as he sits down you hear “DADDDY!!” and if we are kissing hi or bye his son always seems to have a question at that exact moment and kills our moment. The only real time we get together once we get home from work is after Tyler is in bed, and or when he is with his mother. Tyler (son) would never know or see how I feel because I don’t treat him with step child syndrome or anything I just feel jealous inside about the attention and attentiveness he gets from his dad…my boyfriend. It makes me ask myself if I am a bad person for not being the bigger person and understand kids take up all your time until they are old enough to be independent…but I can’t help how I feel.