You’re being too hard on yourself. Remember, the teachers in the videos have been doing this for years and years. It’s also a misconception that flexibility is a requirement to do yoga. Flexibility is actually something you gain, overtime, as a result of practicing yoga. There’s also the fact that different bodies have different amounts of flexibility in different areas because of various physical and lifestyle factors. So, if you can get your arms to 45 degrees above the horizontal, then that’s what you do. If you roll back on your sit bones, sit on a block or blanket (most videos also suggest this). I consider myself somewhere between an intermediate and advanced level student but there are many postures I modify even from beginner and intermediate classes. If you stick with it, you will notice some days the amount of strength and flexibility you have will be different. But most definitely you will notice changes and progress in small increments that will eventually add up to a lot.