These are the years when self-doubt can become prominent. You have many choices to make at this stage in life, and it's normal to feel unsure about certain things. This has been relevant to me recently, and I felt that others in my position could use some support; I know I've needed it.
The answers are not always clear, but you won't get any without asking questions first. Here are some questions to ask yourself in various situations when you might feel doubtful.
When you're doubting a career/major choice...
"Do I enjoy doing this, or am I just good at it?"
"Does this align with my future plans? (The hours I want to work, the type of office I want to work in? If I want have a family?)"
"Do I enjoy my major classes?"
When you're doubting a relationship..
"When I am with this person, am I usually in a good mood or bad mood?"
"What type of relationship do I want with this person, and what does it look like now?"
"Is this a temporary bump in the road, or is this something I cannot get over?"
When you're doubting your faith/spirituality...
"Do I feel fulfilled by my faith/spirituality?"
"Have I been neglecting my faith/spirituality? Is this why I am doubting it?"
"Have my values and beliefs changed?"
When you're doubting your abilities/personality...
"Am I comparing myself to others in an unhealthy way?"
"Am I actively working towards improving this ability/trait?"
"Is there a way for me to develop this ability?" (If not, you have to let it go. Some things are just out of your control.)
What will you study? What do you believe in? Who will you become - and who do you want to become? These questions are not always easy to answer; know that doubt is normal and can be a healthy means of examining your life. Just remember to follow what your answers tell you, and don't be afraid to seek guidance from those you trust and admire.