I've always been a careless emotionless person and now I feel everything getting to me. Why am I like this?

Because you probably suppressed your emotions for so long that now everything is piling up. Allowing yourself to feel things and to express how to feel to people around you that you trust is a great release of built-up emotion, and it will allow you to manage the amount of stuff you have weighing on you at a time. Feeling like everything is getting to you is a common way to feel when you have been used to not feeling typical emotions for so long. However, over time, you will be able to manage this more easily. Expressing your emotions more regularly is a great way to start though!

First of all before I begin to answer this question, I would like to recognize that it is very good to do self-reflection. Mentioning how you have always been a careless emotionless is the first step to healing if that is what you are looking for. Sometimes a person can be this way because of past experiences that have made them change them. It can be because they have been around other people who have also been careless too. Something might have happened that has made you reflect to being mindful about you being this way. I would recommend trying to figure out if something from the past has triggered you.