Firstly, online learning is not for everybody. Seriously. Not having in-person accountability can be a huge problem for some people. Taking individual art lessons or drawing classes locally might be a good way to go. These skills aren't exceptionally rare and people who wield them aren't exceptionally well-paid for doing so.

Secondly, not everybody learns at the same pace, and the course you're following might be too fast for you. Try doing each lesson twice or three times spread out over a few weeks. Some people need a little more reinforcement before it clicks, and they often wind up with better technical skills because they've made 3x the marks everyone else did to get to the same place, on the lesson plan.

Thirdly, you might need more external support. One of the biggest benefits to drawing in a room full of other people drawing the same thing with a professor walking around was hearing when I was doing something well. As you surely know, after staring at a piece for however long, it's nearly impossible to view it objectively, and having someone else saying "hey you did a great job with this hatching there. Maybe you could do some of the line work around this object right here with the same approach." If you don't have that, consider posting in online communities (that aren't toxic as hell) or taking in-person classes.

Fourthly, you might have the incredibly common streak of perfectionism you need to push through. It's psychologically hard to embark upon a task you know you don't have the skill to complete to your standards-- for some more than others. A really good way to push against this is to do a deliberately shitty version off it right off the bat. Messy crooked hairy lines, shitty shapes... just shoot to get the overall proportions approximately right and then maybe the negative space around the edges or something like that. A good quality vine charcoal which is super easily erasable, or a few colored pencils you can use for different preliminary stages in your sketching can really help you out. Counterintuitively, you can also try moving in the opposite direction. Do some fast and loose ink drawings where you can't erase and challenge yourself to finish a drawing no matter how badly you fucked it up. In the end, it's not about being able to do it perfectly, it's about having enough experience to realize how much you can fuck something up and have it still be great. Especially in ink work, those wonky lines and stuff can make some of the most compelling parts of the piece.

Fifthly, if you're getting frustrated with art generally, you might want to start with a different artform first to get the general idea of crafting images, composition, etc etc etc. Photography served that purpose for me. (And you don't need expensive equipment: most of what got me accepted to school was iphone photos and the reviewer was a photography professor. It's the underpinnings that really make something good.) If you're really new to art, generally, this is a really great step. You can't start off at a flat run if you're not confident walking.

Sixthly, be fucking kind to yourself at every possible opportunity. You will usually fail at doing so. It's ok. Just really try. Overall, when I find myself quitting or abandoning something, my first thought is "it's too hard for me" or "man I suck at this." In reality, I don't quit because it's too hard or I suck, I quit because I feel like I've failed at it. It's easy to think you just need the discipline to buckle down and rise above the challenge, but that's quite frequently bullshit. The tougher but better approach, if you can pull it off, is lowering your expectations to a level that's appropriate for where you're coming from and where you're trying to go, while still challenging you to improve. It seems like that would be easier, but it's not, because it requires confronting our egos.

Remember that technical skill is one of the least important parts of being a really good artist. Furthermore, the difficulties you're encountering are incredibly common: this is not a personality flaw, moral failing, or some fundamental intellectual shortcoming, and it doesn't mean you won't be really fucking great. Stick with it friend!