How do I keep it all inside, but not blow up on people?

I've learned from experience that holding things in doesn't make anything better. It actually makes things worse because when you choose to keep your emotions in eventually they come out in a worse way. Mind a way to think about how you feel and then calmly express yourself to whoever you need to. Lashing out can cause tension and even ruin a relationship. But you still need to get things off your chest. Bottling things up doesn't help anything. Try make a list and writing how you feel out then think of how to kindly tell others how you feel. :)