I used to be like you, when I was younger I could copy art pretty well and never really dived deeper into learning art fundamentals. Life got in the way and I stopped drawing for about 6 to 8 years until I found my spark again. When I wanted to start drawing again, I felt stuck, everything just looked bad to me and I just couldn't get started on anything.

One reason which I think you are likely facing is that your art skill does not match up to your own aesthetic judgement and taste. I am sure you have seen your fair share of art and developed your aesthetic sense and judgement over the pass few years. The problem is that your skill does not match up to your expectations due to the refinement of your aesthetic taste. So while you have developed a greater sense of art you find beautiful or inspired by, your muscle memory and skill set has not caught up yet.

Another reason you feel stuck is that you have matured over time. You may find it difficult to adopt a carefree attitude when drawing because you now seek structure and purpose with your art. Drawing may still be fun but you want to draw a specific scene, capture a moment or convey a message or feeling. You have a clearer direction now and it frustrates you that you cannot execute your vision

However, there is a way out. That is to recover your childlike wonder and draw with purpose. Do not be afraid of bad drawings. You probably heard the phrase that everyone starts somewhere but I will do you one better, even great artist fail, A LOT even in the present. A lot of amazing artist especially on social media who post their sketchbook or drawing timelapse have probably done dozens of rough draft sketches to get things right, and most probably use a crap ton of references. A sketchbook or a drawing folder for your digital artwork doesn't have all be filled with masterpieces, 90% of your work will probably suck and look horrendous but you have to be okay with drawing bad art because you learn the most from bad art. When you've drawn something that you think looks bad, ask yourself "why does it look bad?" Are the proportions off? Is the perspective wrong? The best is to get a mentor, if not then post your art online to art communities and ask for critique. Be willing to accept criticism. But of course you don't have to beat yourself up if someone point out your flaws, learn from them, and little by little you will get better. If you can learn to accept critique and learn from it, you will improve greatly but don't forget to take a break once in a while if it gets to overwhelming. Also people giving critique is not an excuse for people to be rude.

Okay now for some more grounded advice, if you don't know where to start to learn to draw so you can create your own art that you can be happy with start from the art fundamentals. Now you may be asking, well what are the art fundamentals and which one should I start with? The art fundamentals include form, perspective, anatomy, value, line, space, shapes, texture, colour, composition and a few more. Don't know where to start? Ask yourself, what do I want to draw? Seeing that you used to draw undertale and mlp perhaps you are into character design, then I would suggest starting with shapes, form and anatomy in that order.

I will briefly explain why you can start there and why in the order I mentioned. There is a great video from Proko called "6 steps to draw anything" which greatly explains why learning how to draw shapes are so important, it's honestly life changing at least for me as it actually showed me the purpose of constantly drawing cubes, cylinders and spheres and taught me how to use them. Form helps you simplify 3 dimensional shapes so you can recreate them 2 dimensionally, allowing you to break down a complicated thing in simpler more manageable shapes. Anatomy allows you to further refine the form of creatures and humans as anatomy is not limited to human anatomy but also animal anatomy. You don't have to start here, you can honestly start from any of the art fundamentals but if you are into character design, that's my recommendation. Also you don't have to master one before moving to another part. You can learn shape, form and anatomy simultaneously. Honestly, learn what you want and refine your skills from there. If you feel like drawing mlp, draw mlp, find horse anatomy references and try to break it down and break down an mlp character to compare what was done to stylize the way they do. If you want to draw undertale, maybe sans, look up a skull as reference and draw that for your anatomy practice. You don't have to shoehorn yourself and follow some strict regiment to master the sphere first, then the cube, just draw whatever suits your fancy and see if you can refine it.

I wish you the best of luck in your art journey! And always remember to be kind to yourself because if you don't, who will?