Since you have been trying for 2 years, you should go see your doctor and see what their medical opinion is. This is only a forum where we can give you ideas, but after 2 years, you have probably heard of and tried everything you can.
A doctor can give you definitive answers. You didn't mention how old you are. You didn't say that you live together. You didn't say if you were ever on birth control. Or if you've ever had anything medically wrong.
Have you charted? Have you done the ovulation kits or are you just guessing when the usual ovulation is? Are you periods regular or are they irregular? Lots of factors go into making a baby.
Just because your boyfriend has 2 children already doesn't mean anything is 'wrong' with you. Go see your doctor.
Good luck!