Your English pronunciation is never going to be perfect. Many times it doesn’t matter if you make mistakes because even though it’s not exactly as a native speaker would say it, you’ll still be understood by the vast majority of native and non-native English speakers around the world. There are many instances, however, which require a little more care. If you mispronounce certain words, you might be saying something completely different from what you expected.
The difference between: sit/seat, did/deed, fit/feet, etc. is a common problem for Spanish speakers. The English “i” is pronounced differently from the Spanish one. As a result, it sounds like Spanish speakers are saying “eat” when in reality they mean “it”.
Check out these videos to hear the difference:
The key is to try and force yourself to pronounce it properly every time. Eventually it’ll become second nature and you won’t have to think about it.