This sucks. For whatever reason your parent isn't there, it sucks. Whether it is your mother or father, it still sucks. Whether they haven't been there since you were born or just walked away when you were older, it sucks. No matter what anyone says it will always be hard for that missing spot to get filled.
What you have to realize is that you are still valid. You are still important. People still care. It's hard, I get it, but in order to move past those emotions you need to acknowledge yourself. You probably don't give yourself enough credit for being the strong person you've become because of that loss.
Wondering why is always the worst part. It's like no answer is good enough as to why your parent isn't there. As a parent it's their job to always be there, to protect us, to give us what we need, but for whatever reason they just couldn't do that. That makes us stronger though. We learn to do things more independently because sometimes we just had to.
Knowing you can't change that parent is also something that can really hurt. What you can do though is change yourself. You can grow from it. If you do have a family, you can be everything they were not. You can't change how someone is. Everyone needs to change for themselves, otherwise it means nothing.
Just because they decided to miss out on your life doesn't mean you have to hold yourself back. Make it a reason to do big things. Remember you are loved and you have meaning.