It's TOTALLY normal and you should feel discouraged for three+ reasons:
1) Difficulty with the poses mean you stand to benefit the most. You are exactly who needs some basic strength and flexibility!
2) Progress typically follows a logarithmic-like curve meaning that you make a lot of progress early on. Stick with it for some consistency and you will be motivated and encouraged by how much more you can do in a month. It will be a lot more. But only if your practice is consistent.
3) You are 20, which means you body is more responsive to exercise and mobility training right now then it will ever be the rest of your life. If you want to get ahead of this and not become a 50 year old who aches standing up from a chair and has to tense up and grunt to gingerly pick something up off the ground and can't run around or horseplay with children or go hiking on a nice day, then starting now is the absolute best option.
Keep in mind that many of us here (including me) were in the exact same situation you are. Also, keep in mind that you don't have to reach the poses - you can modify them and also stretch where you notice you are too tight. For example: if you can't reach your hands past your knees, stop trying to forward fold. During that portion of the flow just hang out in a gentle hamstring stretch. When Adrienne tells you to come up halfway, then close the space again you should feel confident to ignore her and just gently stretch.
Fell that way about all of the asanas. Do what you can and confidently flirt with your boundaries without worrying about "nailing it".
Good luck!