I can very much relate to the feeling of inadequacy, especially when comparing myself to others. What helps me get through these slumps is to concentrate on the things, I can be proud of. And you have something there to be proud of! Your comic has reached an audience (150 subs? holy sh....I'm jealous!). So even if your artwork is not up to your standarts yet, a lot of people see the potential you have and are interested in what your story offers. That is something not many people can say about themselves. You also got fanart, which is a clear sign that your character designs are interesting and cool.
So.... just build on that. Others have already said it: You'll get better with practice. Try to remember that you do have a solid foundation and give it time. Try to change your perspective and focus on your own accomplishments instead of the success of others. You should still strive for improvement, but this change of perspective can offer a more healthy way to deal with your own goals. Not "Everyone is better than me and I'm no good", but "I've already come so far, now off to getting even better!"
And if you see someone doing better than you: try to learn from them. They have started with stiff action sequences once as well, so they know how to get better at it ^^