I think ... is used by a lot of natives. Other options are, In my opinion ... I believe that ... It seems to me that ... To my way of thinking, ... As I see it, ... I feel that ...

Firstly, if you are saying it in the negative, as in your headline, then it should be: "I don't think I want to..." rather than "I think I don't want to..." Secondly, there is nothing wrong with "think," it sounds very native. However, some alternatives are: "I find that I don't like going to the movies as much as I used to." "I feel as though I'm starting to enjoy my English class a bit more." "I reckon he's the best player in the world." (Though 'reckon' is quite Australian; I'm not sure other English speakers use it very much.)

Thank you !

You can say: "I believe....", "I presume....", "From what I can conclude,..." , "I feel...", "I found that...", "In my opinion..." ***But many people around me who are native English speakers still use "I think" a lot, although it's not very formal. So you can try saying the ones I states before..Those are bit more formal^^