How can a person stop himself from being some thing he doesn't want be?

Hey! From my philosophy, we are all the abstract product of our environment -- albeit we aren't confined to that shape, but we must recognize it. Acknowledge it. Now, while you're "beginning" shape has a form that you didn't necessarily dictate (no one really chooses their origin/family/socioeconomic background/race/etc), your "final" shape can be defined by you. And of course, crafting a "person" from a shape isn't easy and I won't say it's not going to be painful at times to grow in new directions, but that's what makes the final product so beautiful. Just by asking this question, you are demonstrating at least a willingness to try and I applaud you for that. As for the remaining leg work, it's going to be a whole lot of internal discipline -- in a system that works for you. If you want to talk about this privately, message me and I'd be happy to converse. All the best!

By training our self-control. Different people have different things that tempt them the most -- some people are very easily tempted by greed, some instinctively focus only on the "sexiness" of the bodies when they see others, some may find it very hard not to be a couch potato the whole day, some may find it hard to stop thinking about an obsession and focus on their loved ones even for a minute -- and self-control doesn't necessarily come naturally to us, proven ever since our birth. It is something that we need to keep training for. Just like we discipline our physical by exercising routinely in the gym, likewise, it takes efforts to discipline our interior. In every moment of encounter with a situation that calls for our decision making related to that something, apply our strong will and move away from that thing that is luring you; slowly, it will get easier every time we manage to successfully say no to it. Take one step at a time. Usually, if there's something nudging us in our good conscience, it's a good call to reflect and assess that thing. One way is we could contemplate on its source; it could lead to our weakness, our past hurts, or the big questions of life itself, so it's always important to pray for graces along the way.