Whitediamond, forums are difficult because readers can't tell whether you are posting only to express your thoughts an let them out without ever considering letting them impact on your every day decisions, or whether you are posting to get encouragement that you are entitled to act on your thoughts.
If the former, then of course, that's absolutely fine. We all have thoughts of how our life would be better, but we know there are selfish thoughts so we don't act on them. Letting it all out does feel good.
However, many people will seek approval to act on their selfish thoughts and that's when it's not ok most of the time.
You are young and as such, it's inevitable (and therefore NOT a criticism) that you will think and make decisions that are less mature than an older woman, or really one with more experience. For instance, when you say that he didn't have his kids weekends for two years, it was right to assume that he wouldn't continue to do so. This was not correct. When you get with someone, you accept that at any time, the status quo can change. It impacts on everything, jobs that are lost, homes that need to be vacated, illnesses that require a change in your lifestyle, older parents you need to start supporting AND children who you need to prove more care for. Yourself introduced a change in the life of your step-children by giving then a sibling.
In the end, only you know whether you, your partner and your SC are happy with your situation overall, even if it means that some adjustments might need to be made. If the fact that your life is not exactly how you wish it was makes you unhappy, then you need to focus on moving forward, not holding on to what it could be if..... Living your life on ifs will only make you and your family miserable.
So yes of course you can have a moan and if you can't do this with your friends, then here is the right place, but it would help with responses if you made it clear that you don't have any intentions to act on your wishes, ie. you would never consider putting pressure or worse demanding that your OH to refuse to have his children at the weekends because it would make your life better, even if it means him and his children not being so.