How to learn Muy Thai or to fight really well by myself? Somewhere online like YouTube preferably? so i can be good fighter
There's some things you can learn by yourself, and some things you can't. Being a good fighter is definitely something you can't learn alone.
With that said, here's some things to keep in mind:
You don't necessarily need a full fledged instructor
You do need someone to show you how to move, who is skilled enough to know the correct movement, timing and can give you tips on how to get that and the power you need. That doesn't necessarily have to be a teacher - that might be someone who's done it for a while. I've seen instructors teach poorly or in dangerous manners, and "students" who have taught really well and have the knowledge for safe training. It really varies.
Things you can't see
Not in some magical sense, but some things like feeling if the impact is right, balance, or shifting of weight are often things you have to feel, and have someone who can tell you what you need to do to get there. This is the main reason you want someone more experienced to show you the way - you might be training something you think looks right, but doesn't get the power or the force you need.
Whether you're talking sport fighting or self defense fighting, there's going to have to be actual drills and sparring with a live person. You have to know how to hit a person, how to take a hit, how to read the body language, feel positioning, timing and adapt. You can't do that by yourself. If you're looking to train self defense, you're also going to have to train to deal with weapons, multiple attackers, and extreme stress situations - things which you can't really do alone.
The bits that you can train alone
After you know how a move should feel, you certainly can practice it over and over on your own time and make sure your delivery is good. You can practice conditioning: strength, endurance, flexibility. All of this is stuff you can do on your own time, but part of it is that it alone, won't make you a good fighter.
If you can't train with anyone else at this time, you can always focus on doing lots of strength and cardio conditioning. It will put you in a good position for when you do finally get to work with someone who can show you in person stuff to work with.
As the consensus has already stated, you can't learn on your own, at least not in the beginning. There are just too many things that you need someone to actually show you.
Once you have several years under your belt, typically about 5 solid years of learning, depending on the art 2 of 3 can suffice, then you can begin to learn from instructional videos, books and even good martial arts movies, because you'll already have a solid foundation and be able to simply incorporate new techniques into your fighting.
But without that solid foundation that only comes from being taught in person by a good teacher that can you show you your errors, put you in the right positions for blocks, stances, kicks, strikes, grappling, escape techniques and to help you to get the FEEL of a technique, something that can never be learned from a book or video, you will simply not get much of anywhere and you will waste years of time, years of effort and probably a lot of money trying to do things on your own as you won't even know what videos or books to get that are or are not good, even if you could learn from those alone.
You can't! There are lots of stupid videos out there that are run by money grabbing organisations. Can you play the guitar without picking up a guitar? No! So you need to get down to a gym and learn.
I've seen people at the gym hurt themselves badly by kicking a heavy bag without any technique. People often come up to me after one of my heavy bag sessions to ask for tips. I tell them to go and get 27 years of training under their belts and then they'll begin to appreciate what they are actually doing.
You can invest in all the right gear but you'll never get anywhere. It's like trying to play golf without having had any lessons. You could be a natural and get around a course on par (unlikely), but you'll never be playing with Tiger Woods!
I've seen many bad instructors too, so do your research in advance. Look at their videos online beforehand to see if they kick well or ask someone who can spot a good kick to evaluate for you.
It's not possible to learn a martial art at home with no partners. Even if you had partners and the necessary equipment, you don't have a knowledgeable instructor to point out your errors. You need someone experienced in the art to notice and show you when you're doing things wrong, to schedule your training, to push you, to arrange sparring matches and appropriate competitive match-ups, and to hold pads for you effectively. You can't do it yourself.
You need to find a quality muay Thai gym and train there at least several times a week if you want to learn muay Thai for fighting.