A 2.5 year old isn't capable of rational thought or plotting as in ' if I behave like an adult and don't speak or be demanding I won't put pressure on my pregnant mum, who is tired, and then perhaps she will like me more'
Maybe you are tired and a bit anxious and not as patient and as fun as you have been in the past, but really your 2.5 year old is being a 2.5 year old.
They assert them selves and push the boundaries at this age.
What is going to happen to you with a new baby coming is going to be hard. But however hard it is , it is going to PASS eventually and both babies, ( because that is what a 2.5 year old still is) are going to rely on you for their memories of childhood.
You are going to need lots of patience. Lots of remembering that you are the adult and responsible for their emotional development.
You sound tired and exasperated with your 2.5 year old and we have all been there. I felt like throttling mine on occasion. However the more you count ten, listen, and treat them with respect the more you will be rewarded in shed loads when they are older.
They WILL remember how you treated them. They may not remember the words but they will remember the feelings.
So, sorry, but it is down to you to change how you feel about your DD.
Try and relax, and enjoy her and her company in the short time you have got left, with just her. If you sigh and tut and frown, she will be picking up on the fact that perhaps you don't like her ( I know you love her and I am not saying you don't) it will make her even more clingy.
Just treat her in the same way you would like to treated and you can't go far wrong.
You are a good Mum I am sure, but you have one go at bringing them up to adult hood, so that they can face the world with confidence. It takes lots of patience , because there is NO going back and doing it again.
Best of luck with the new baby. X hugs