If anyone can fix your black thumb it's the members here. They are doing a fantastic job with mine.
Knowing the identity of your plants
is a very good start. Now some other information that would help so they can identify the problem.
PH of the soil?
How much water are the plants
What kind of fertilizer and how often?
How much sun do they get everyday and is it morning or afternoon sun?
Can you believe all of this makes a difference. I didn't know about any of it when I got here in March but I'm learning.