Hi Simone!

It's completely normal to feel emotional frustration, mild physical discomfort and a lack of coordination with spider legs exercises done correctly when you are a beginner. It's because we are using a set of muscles and forming new memory for them in previously unused unfamiliar territory.

Don't overdo the stretch extensions. Start easy, little and often, increasing with time as it becomes easier. Christopher Schlegel has some warmup stretching exercises you should use as well. I'll see if I can find the link in a moment. There you go. Linked above.

Everyone has different flexibility characteristics too depending upon their body type. Taking this into account, I could never stretch in warmup exercises in either Judo or Karate when I was a young man like some of my lithe bodied ectomorphic contemporaries, but neither could they match the speed or power of my punches, kicks or throws. We have the tools we are gifted with by birthright. The choice lies in what we choose to do with them, and how we use them applied to that choice. In playing guitar though, my observation is that being born with ectomorphic build and flexibility is of significant advantage.