Please correct me if I've made any mistakes.

I've no doubt that my past experience was very useful to me as I learned how to communicate with people, how to be a hard worker (or better: "how to work hard"), be patient , be an effective member in of a team, work independently, be a good thinker (though "think clearly" is better) and be much more motivated. I also gained skills on how to manage in managing my time (and hence my workload), in handling enquiries and problems in a professional way and in dealing with hard ("difficult" is better than "hard" here) situations. This was not just useful in my work but even more in also my life.

That's the minimum to turn it into "correct" English (if such a thing even exists), but here's how I would write it myself, with shorter sentences, and rearranging the words somewhat.

I've no doubt that my past experiences have been useful to me. I know how to communicate with people, and how to work hard. I've learned to be patient, think clearly and work effectively, both within a team and independently. I'm also much more motivated.

I've gained skills in time management and am able to handle enquiries, problems and difficult situations in a professional and confident manner.

These skills, while certainly useful in my work, have been even more useful in my life.