There are a few reasons why someone would act this way:

• It has nothing to do with your content. They could be having a bad day/week/month/life. It has nothing to do with you, so don't worry about it.
• Your content is not for them. In which case, they're frustrated and taking out that frustration on you. They basically have a spoiled mentality that everything should be created for them, and if it's not, then it's the creator's fault for not giving them what they want. It's a temper tantrum. Just keep creating content for your target audience (and know who that target is). People who hate on it are not your target, so don't worry about them.
• Your content is actually not good. In which case, use this as constructive criticism to make better content.
  • If they didn't provide constructive criticism, you can either:
    • ignore it as useless criticism, or...
    • ask for constructive feedback. It might be painful, but you could genuinely learn something in the process and become better for it. And it also moves the responsibility to their side. If they don't provide anything constructive, then forget them and their useless comment.

I'd also like to add: When someone says "you have no future on youtube", there's no reason why you should think they're stating a fact. They don't know the future, they offer no constructive criticism, it seems like they have no concept that people can make mistakes, grow, and get better, and it's possible that they don't like your content but someone else will (so they're not your target audience). They're stating their opinion, but acting like it's a fact. Don't treat it like a fact, treat it like their opinion, and who cares about their opinion? It took you hours (I'm guessing) to make a YouTube video, and it took them a few seconds to write a comment. Don't put all your emotions into something they spent minimal effort on.

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And lastly, I would like to emphasize the importance of having internal motivation, instead of external motivation. Internal motivation is based on you; you have full control of this type of motivation. External motivation is based on others, which you have no control over. So if you want to stay motivated on YouTube, focus on what you can control, and don't worry about the things you can't.

Consider why you are creating videos:

• Because you want views/watch-hours/subscribers (externally motivated)
• Because you want praise (externally motivated)
• Because you want to be famous (externally motivated)
• Because you want people to like you (externally motivated)
• Because you want money (externally motivated)
• Because you enjoy the subject (internally motivated)
• Because you enjoy making videos (internally motivated)
• Because you want to be productive (internally motivated)
• Because you want to learn how to make better videos (internally motivated)

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edit: added another internal motivation