Selfish is defined as lacking consideration for others -- concerned chiefly with one's own personal profit or pleasure. Many people nowadays see this as a negative trait someone carries and rightfully so. But, sometimes it's okay to be selfish and do what is best for you.
I will say that sometimes being selfish can come at the expense of other's feelings, but that doesn't make you a terrible person. It just makes you human. Everybody has the natural desire to do what's best for themselves -- some more so than others and that can be seen negatively. There have been countless times where I sacrifice my own happiness just to avoid hurting someone else's feelings. While doing this, you may have all the best intentions to the other parties involved -- the only one who'll end up disappointed in the end is yourself.
There is a time and place to be selfish, as well as selfless -- just make sure you know which is fitting for what situation. I know it can be confusing -- you want to obviously do what you would like, but you may want to make the other person happy as well. I'm 100 percent guilty of this and I'm sure many people can relate. At the beginning of last year when choosing where I wanted to live for my senior year of college, I was completely torn between two options. Live with a couple of friends I had gotten closer with or stick with my roommate I had been with for the past two years. Long story short, I ended up compromising to avoid hurting anyone's feelings and I regret it to this day. That is the perfect example of when to choose yourself and be selfish.
In the end, you're the one who has to live with the decision you make -- nobody else. Don't screw people over in the process, but be mindful of what you want and what you think is best for you. I know it can be difficult, but you'll end up much happier in the end.