Today, I had a presentation. I finished preparing it at the last minute because I had started too late. As a result, I couldn’t present it well. I felt a bit disappointed, but it was a good lesson. Next time, I’ll start earlier and prepare more carefully. I’ll do better! You made no actual grammatical mistakes. I question your use of the word "lesson". It has two possible meanings here, and which one you intend is not at all clear. The "lesson" could be your presentation; in other words, you presented a lesson. However, it could also mean "learning experience". In the latter case, you would say "it was a good lesson for me" or "it taught me a good lesson".

Today, I gave a presentation. I finished preparing it at the last minute because I started too late. As a result, I didn't present it well. I felt a bit disappointed, but it was a good lesson. Next time, I’ll start earlier and prepare more carefully. I’ll do better!