I'm Punchie. So I've been speaking English for years. Now I use English everyday more than I use my native language, so I'm very fluent. I tend to speak quite fast and there'r barely any communication breakdowns. ( The breakdowns usually caused by cultural differences, I think...
But I make loads of mistake, pronunciation, grammar and sentence structure. And I don't know what to do. I think the problem is I speak too fast and I don't think of all the rules before I speak.
Is there any trick or lesson out there that would help? I really want to fix this, it's kinda embarrassing sometimes at this point...
I think you would benefit from a tutor or language partner that would correct your mistakes as soon as you make them (as opposed to most people during conversation who, trying to be polite, will only let you know when the mistake is serious enough that there is a misunderstanding). If you have been speaking English for years, most of your mistakes are probably related to habits from your first language that have never been eliminated and are somewhat automatic. My guess, based on my experience, is that without correction, sometimes you yourself are aware of the mistakes (right after you make it) but often it is so habitual that there are times you do not even realize you are doing it and the habit continues.. (I have needed this from my Spanish teachers. As a native English speaker I have a tendency to say the schwa instead of the ¨a¨ or the ¨o¨ if it is unstressed at the end of a word. Even though I know I have this tendency, I have needed my teachers/partners to point this out to me frequently to reduce this habit.)
I think you need to be clear with those helping you that you need them to be picky. Then as they are picky, try to slow down a bit. Consider youself practicing speaking perfectly. Hopefully that practice will then slowly generalize into your speech habits when you are not with a language partner or teacher. If you can find language partners with good enough English skills to know when you are making mistakes and are willing to point them out to you, I think they could do as much for you as a professional teacher. I think you would just need quite a number of hours doing this for practice, so language partner or one of the less expensive teachers with the right personality to be willing to stop you in conversation when you make an error would work best.
I agree that you need to slow down and be aware of the exact words you are saying. Think of a musician who needs to play a very fast and complex melody. In some parts she fluffs some notes (ie. plays them badly, or even plays the wrong notes). So as a good musician, she looks at the parts with the mistakes, slows down and plays those parts separately, correctly, slowly and repeatedly until it's a habit. Only then does she speed up.
I'm sure that you don't consciously think of all the grammatical rules in Thai when you speak; you just know what's appropriate, and you choose the right words and grammar out of habit. So aim for the same in English.
If you want some extra feedback, you could record yourself speaking (30 seconds to 1 minute at most) and post the link in this discussion. Members here often use vocaroo.com or clyp.it, but of course you can also use youtube or soundcloud.