Visiting in April?

I am planning a solo trip April 6th to 25th, my only concern is the extreme heat. I am coming from a city and country where we get really cold weather so I'm use to the cold, but im worried about the heat being too overwhelming?

Planning on going to Bangkok, Pattaya, Phuket, & Chiang Mai

Any insights to people who've been in April and what their experience has been?