Losing a parent in itself is one of the most difficult things a person can go through in their lifetime. At least that's how it was in my experience. Losing one of the people that was supposed to be there to take care of you, that was their purpose as a parent, to take care of you then when you need it the most they were gone. And to lose them young is especially difficult because you are in the midst of growing up, thats when you need support the most from your family.
You probably don't go a day without thinking about them. Normal day to day occurrences aren't terrible, it gets easier as time goes on but holidays can be terrible. Mother's Day and Father's Day are obvious ones but even things like Fourth of July can be difficult when you recall memories with them helping you set of firecrackers in your old driveway. But everyday thoughts range from things like, "What they would be doing now?" or "How it would be to call them up and just chat about your day?" -- you know, normal stuff about homework and friends, talk about March Madness or about a the election. This is a privilege that most young adults that haven't lost a parent don't think of. Encourage those people, your friends, to cherish every second they get to have with their parents. Enjoy hugs, free meals and parental advice because you never know how soon they will be taken from you.
Losing a parent causes you to grow up at such a rapid rate. You learned loss early and you no longer take your time with family for granted.You know how precious it is to be with your family and more importantly you gain so much strength from this loss. You become closer with other family members and rely on them by helping each other through the grieving process, which may last for months and months. You know what is important in your life and you know what you should take seriously and what you shouldn't take seriously. You learn loss and strength and how to be resilient in your life. Because ultimately you just want them to be proud of you. In the back of your head you know that they would be proud of you no matter what you choose to do.
You could spend all day talking about the unfairness of your situation. You could be bitter. They don't get to attend your future wedding or be there as you walk across the stage at your graduation(s).They didn't get to live a full life or see their future grandkids. They didn't even get to be there as you yourself grew from kid to adult.
But in this life you have choice. So choose to be happy. Choose to remember them in happy memories and value the time you had with them. Stay strong. Don't look back with bitterness in your heart. Take heart in the fact that you knew them and that you were able to learn from them while they were alive and learn from them after they have passed. You are strong and you can be happy with them guiding you from whatever heaven they are in.