I am afraid my partner will change from positive to negative

My partner is very sweet and caring, always helping me and making sure i am ok and i have everything i need. We have been together since last december. This is the best relationship i have ever had, but lately i have this fear that he is hiding his true self and my worst fear is that he will go from sweet and gentle to treating me badly.
I think this is because i watched on YouTube videos where the women talked about their abusive relationship saying their partner turned into a negative person. I watched many of them and now i am scared this could happen even tho its been 7 months and my partner is the same like he was in december.

I think you would have seen some red flags by now but you haven’t. Try enjoy it for what it is without trying to read too much into it.

Have you ever disagreed with him?

Not really, we havent had big fights or fights in general, we are both very calm and if something bothers one we said that in a calm way.

Maybe YOUR the negative person your afraid of. Why are you watching negative YouTube videos ? Why don’t you watch positive ones. Why get worked up over something that hasn’t happened yet ? Your statement about how you have a “fear that he is hiding his true self “ is very telling. Why do you have this fear ? Has he done something to warrant this fear ? And why do you think that he will change his personality. Well that happens to MANY people over time. During your life , whether your with this particular person or not you will learn , if you haven’t already , that people can go from love to hate in a millisecond. You just need to know this and be prepared. Hope things work out for you.

Did you used to be in an abusive relationship? If so, it would explain why you're watching these types of videos on YouTube. I agree with the above. Unless you're seeing or have seen red flags, there's no reason to worry about something that hasn't happened yet. ENJOY the relationship. It sounds healthy.

There are certainly no guarantees about the future with regards to any relationship.

Still, it is interesting that you have this fear emerging now. Has this happened to you in the past? Or did you see it happen around you? I would explore this fear with a trained professional. It might provide insight into your patterns of behaviour and/or relationship dynamics.