Of course you can live off of 1800 USD in the PH. I used to live off of 400 USD a month there too. I even know an expat who lives in a Filipino village who lives off of 100 USD a month, amazingly.
You can live off of any amount in the PH. It's just that if you are living off a low amount, like I did, then your qualify of life will go down.
If local Filipinos can live off of 200 dollars a month, or less, why can't you? You can live off any amount, but your quality of life will vary.
Learn to cut unnecessary costs, and don't overpay or lend money to anyone. Spend efficiently. That's the key.
I now live off of 1800 USD a month here too, and am able to spend more freely than before.
Why do you want to move from Thailand to the Philippines though? From what I heard, Thailand beats the Philippines in almost every category - price, quality, culture, etc.
Also, beggars don't come up and touch you in Thailand like they do in the Philippines, which is scary and intrusive. They sit on the street and wait for donations, like in the US. As we all know, there are many negatives in the Philippines that over time become annoying.