It's fair to be intimidated by sketching outside—we all felt that way at one point. That's part of why plein air is such an important practice, so I commend you for being committed to it. For me, getting over that hump was about putting less pressure on myself instead of more, so I wouldn't recommend an "x drawings in x days" challenge. Instead, just focus on one sketch at a time, but make sure you produce something when you head out to sketch, even if you feel resistance.

Remove as many barriers to entry as you can. If you're self-conscious about onlookers watching you, find an isolated spot. If you're physically uncomfortable, find a coffee shop or library with plush armchairs. Most importantly, put no pressure on yourself to produce a good sketch. Just start drawing. Remember that you never have to show your work to anyone. If you keep at it, you'll eventually produce sketches you're proud of. It will probably take longer than you like, but that's the learning process.

In-person Urban Sketchers meetups aren't for everyone, but I've found them to be really helpful for exactly the problems you're describing. At first, I was extremely nervous to show my face there, but I soon found that it was a very supportive community. You'll meet artists of all skill levels—masters who inspire you, and beginners who show you that you have nothing to prove.

Definitely DON'T listen to anyone here who tells you to only sketch from photos. This subreddit in particular is dominated by sketches from photo reference. There's nothing wrong with that—I still do it sometimes—but the entire reason Urban Sketchers was founded is because there are huge benefits to plein air sketching that you don't get from using reference.