My DSD is twice your DSS's age, so obviously that makes things a bit different....
having said that, though - I DO relate to what you say. My DSD is very different from me, and some of her personality traits really clash with mine; some of her behaviours are so far from what I (rightly or wrongly) am used to, that I do find it irritating. And if I am honest, I too am struggling to 'accept' her for who she is. DSD is a very dominant personality, and she also has a profound need to control. This results in some simply annoying behaviours, like always wanting to decide what we eat for dinner and when, or always having to hold the remote control, or rearranging the sitting room furniture when she decides she's ready for a change, or needing to constantly direct and dominate every conversation! I am not as confident or outgoing as she is, and I often end up feeling a bit bullied by her. The biggest problem I have is that she wants to be the Woman of The House, and be her dad's partner and a sort of mother figure to the other kids - which absolutely drives me insane.
I am sorry that you have taken such stick for writing about how you feel, because I think it is both natural and understandable to feel that way, and as I say, I do relate. I also have to agree with allnewtaketwo - feelings are not WRONG. Actions can be wrong.
Are there any positive actions you can take to reduce your feelings of annoyance and help you embrace this kid as a part of your life? Well, that is what I ask myself all the time. Some days it is easier to do it than others. In my case, it has been 3 and a half years, and very much a 'two steps forward, one step back' kind of thing.
For me, the one thing I have found that really does work is when I ACTIVELY make CHOICES to include DSD more in my life - asking her to do something that she's not usually encouraged by me to do, or asking her to spend time with me when I start out feeling like I want to run away and hide in my bedroom. Maybe it is a version of 'fake it until you make it' - or maybe it is because then I am the one LETTING things happen, instead of feeling like things are happening TO ME, beond my control...I don't know. I can't do it all the time, because it is hard to let go and hard to reach out to her just when I most want to be left alone! But I have discovered that at the times when I am most annoyed, the one thing that does work is to actually do something that scares me a little.
This might not be a very good example - but - she and I both like to bake. I get really wound up when she does, because she always creates a huge mess and leaves it, or uses my nice (expensive!) pans and scratches them, or uses the last eggs when I was planning to cook breakfast the next day...you get the idea. My DH thinks it would be marvelous if she and I baked together, but I tried that early on and it was a disaster, because she just bossed me around the whole time and I felt really irritated. So, that idea of the two of us happily mixing up cookie dough together is probably a bit too much! It also turns too often into a competition for dad's affection (who is going to make his birthday cake? whose cookies does he prefer?) or a chance to remind me that I am not her mum. :(
But. She is very artistic. So on her brother's last birthday, I made the cake, and asked her to decorate it. I didn't wait for her to ask or insist on it, I asked HER. And she did a great job, and it meant we both had a part to play, and she was pleased to be asked and I was pleased because I got to do the asking. I thought it would be hard (because it would spark an argument, or because I'd feel displaced) but instead it felt really good. Now we have rules around the baking (don't use my pans; clean up after yourself), but we also can share recipes or look at cookbooks together.
I should add, that this technique works best for me when it has NOTHING to do with DH, it has to be just DSD and I doing something together.
So - sorry for the long post - but that's my advice, I guess. Try to do things alone with your DSS when he is there. And especially, try to think of ways you can involve him with the new baby - figure out what you CAN be comfortable with, and know your limits, but use it as a chance to do things one-on-one with him...maybe you and he could make something together to give the baby when it arrives, a project that you could work on together over several visits or even longer.
You might also think about whether the expectations that you and your DP each have - about your stepson's behaviour - are clear and reasonable. Do you guys agree about how to handle it when he picks at his food? Are there rules that are fair for an 8-year-old kid? If not, can you make some, between you?