I know this post is old but maybe someone reading it can be helped by what I'm about to advise.
I jus turned 32, and have lived a very uhh vivid life lol. I'm an orphan, foster kid, adoptee, American immigrant, runaway, felon, lifelong minority, gun violence survivor, suicidal failure, recovering addict/alcoholic, currently non active gang member, and many more stereotypically negative identities but also a man who has painstakingly suffered to gain his wisdom through many experiences and now knows how to use and learn from that negativity, as well as a new father and I guess a step father relatively unconsciously for about 5 years now.
My step son is 9, about to be 10 n I've known him since he was 4. His father left him at about a month and never came back (me and him have that in common except my pops left when I was a fetus lol then my mom left at around the same time his dad did). For him tho his dad is a few hoods away with 2 other sons that he raises but doesn't claim his first born, who is now mine so fukk him. My daughter (by blood and reason my life changed) is his sister and is 2 now and me and his mom live together as strictly roommates but aren't together (complicated I know but more for financial reasons and so I can see my daughter as much as I want lol). Anyway, I have much experience with this subject and have learned to fail to succeed. Many who commented are probably not doing so from the perspective of a step parent but more then likely a step child so some negativity is to be expected due to their own negative experiences and justifiably so, this is how your step son most likely feels multiplied by personal experience.
For me though, I have both perspectives as a step child, foster kid, adoptee, adopted step child as well as a father and step father who's gone and is still going through similar challenges but I have found the peace and harmony u speak of, which in actuality comes from within myself instead of a goal to be met through outside environments.
So I can not only relate but offer advice and at the very least let u kno what ur feeling is normal and u are not alone.
First it's important to state that love and hate are not opposites but different degrees of the same thing. Much like life and death, hot and cold, light and dark. This means that ur feelings are justified and don't need to be ashamed of and you actually are in a position of positive growth more then it seems or u think. You jus need to find ways to shift your mentality and focus more on your own issues then his. It would be much worse if u felt nothing for him at all. If u felt nothing I'd tell u to get a divorce straight up like that.
U have a hard task ahead of you though, but by now your step son is grown. Maybe u have already gotten a divorce or have become the mom u wished, to someone who doesn't share your blood, which trust me is possible, I should kno.
It's also important to note that this is an issue having to do more with yourself then him although I completely understand how u feel. I been hated my step son and still do sometimes. We have had many battles trust me and will have many more I'm positive. The main thing is to learn how to fight and with what tools. Tools to grow, not destroy. This doesn't mean that it won't be ugly, it will be terrible at times garaunteed. N by the sounds of it u were like me and hopped into a relationship naïve and inexperienced with love itself but again jus like me is the time to expand your soul to another depth. It will only help u as a mother to your child as well as your step kid.
My step son can b a know it all without knowing anything, struggles with social behaviors at school like hitting girls and still has no close friends going into 4th grade this year, he is sort of dramatically pathetic at most things because he isn't coordinated or even healthy most of the time and isn't good at things "regular" kids take advantage of like riding a bike, he can't swim, play sports, which is fine n we are working at it but what bothers me is he doesn't try and expects things to come to him without any work put in much like the video games he is constantly playing smfh!!! He's also a RIDICULOUSLY picky eater, I mean he will start dry heaving at the SIGHT of a carrot (tru story on more then one occasion smh) and wastes a lot of food which I hate because we are literally poor since I got shot a few months ago then got out of jail for a warrant I had from a couple years ago (karma from my past n hopefully is a paid debt now, fingers crossed). Btw, being poor is fine, it sukks but everything has its negatives and positives, it jus depends what u focus on. Besides I hear being poor is suppose to be the new fancy trend..."minimalism" ryt? Lol.
Anyway, it's natural for the bond between you and another human being who u are not directly related to, to be hard to gain. As mammals and forms of life, one aspect of survival is helping our own genes survive and seeing others as competition. This is why many aren't quite sympathetic to others struggles and need a bond first before they can be empathetic to someone elses or that parents prefer their own child to someone else's. But it's less physical then mental so is definitely possible to feel like a parent to children not of your own blood. Nature still wants the strongest to survive but strong is always within the context of that era or culture. What's strong one day to one person might not be strong the next and to someone else so life knows to have many different forms of life with all different type of traits to give the best possibility of survival for life itself to keep exiating.
It sounds like the problem is more within yourself, having to do with your own insecurities about people respecting you or liking you, perhaps something having to do with your spouses ex? Idk but I will need to think on these things deeper to form a strategy that will work the long way. Fake it til u make it is a good start but only works to form new habits temporarily so don't fake it forever. I could be wrong of course but I've learned over the years that when people hate other people (which I am VERY familiar with at both ends) it more then likely is due to how they hate themselves in some way. A quote I made goes like this "I met an ignorant man and we began to fight. After, I remembered a time when I too shared in that ignorance and suddenly wondered who I was really fighting with?" It basically means that our own insecurities are mirrored or projected onto other people. Usually judge mental people are miserable inside because in the same way they judge others they judge themselves although usually only in their subconscious.
Another example is like If I hate how I look, and have taken steps to be physically beautiful as an overcompensation to that insecurity, I will judge others who I think are "ugly" because I am really disgusted with myself and they remind me of the thing I think is disgusting about myself, again usually in my subconscious. Does that make sense?? So personally whenever I get really pissed at my step shorty, I try and reverse that hate to myself or mirror it back and then try and understand the real issue. Like today when I spent an hour making a bomb ass breakfast and he ended up not eating and looking at it pathetically without swallowing the tiniest bite and not even tasting it really, I just thought how much I'd like to whip his ass, called him an unhealthy weirdo and that he was guna get fukkin diabetes soon and that wen he does ima jus say I told yo lil ass so!!! So, while this was definitely not my proudest moment, it was important to express my anger (hopefully better next time lol n there will b a next time). Then, after looking up other people's experiences and gaining back perspective, I kno I overreacted, took this time to get away from the situation (which is key) and regroup and regain balance. The main thing is I got back ryt with myself and as an added bonus he even apologized to me when he got back home with his mom and I saw that he really doesn't do these things just to piss me off. That's how selfish and over sensitive we can be as parents sometimes tho. So specifically U need to start taking time for yourself to give yourself what u need, weather that's a break, respect, love, compassion, discipline etc and u will find, if u can b there for yourself, u won't need others to do so for you so if they dont you aren't resentful and if they are, it's just an extra bonus. I'd need more details as to your exact situation but just tried covering the broad stokes but I hope you or someone reading can find the help that I did through this, which is the skill of getting the help to help ourselves. Love from Chicago