Mummy guilt :(

Hi Mums. I am a mummy of two beautiful children. One is 2 1/2 the other is 2 weeks old.
My eldest is a very gentle little soul and a very well behaved child. When the new family member was born my 2 1/2 year old took it really well. There was a couple days where they were a bit out of sorts but all seemed to settle. Until the last few days we have had waking in the middle of his day sleep screaming and crying being really hard to settle :(
And in the day we have had attitude and huge meltdowns to the point I walk him them into the bedroom and lay them on the little kids couch with the door left open and say come out when you have stopped crying and can use your nice words. Nothing else calms them.
This is breaking my heart knowing we have just brought a new little addition and a bomb shell to their little world. It's so hard to see them like this. When baby is asleep we play and do things together and even when I am feeding I am right with them whether it be sat on the back patio or on the floor doing puzzles. The eldest is very involved with helping with the baby when they want to and the eldest is never nasty towards the baby. I know it will pass, I hope.i just want to know I am not alone I guess :( I am a very hands on mum and do a lot with my kids. I do the cleaning and dinner prep at nights so my days are spent with my kids, they grow up too fast and I found this suits our family the best. Please help, I feel like I am just doing something wrong. Will I ever get my first child to Back how they used to be :(