It's bloody hard.

[melisma]

Just that. I have DS who is 2.5 and DD who is 14 weeks. Today I have just felt so impatient with my son not listening, running off etc and was way more shouty with him than i want to be. He is so great but just never stops! also feeling constantly guilty that I'm not meeting my daughter's needs properly either. Please tell me it gets easier. ..

It does! This was me 3 months ago. DD2 is now 6 months and while weaning and crawling make things trickier it's not nearly as difficult as it was.
I realised afterwards that DD1's very challenging and defiant behaviour was brought on by a combination of new baby jealousy, tired/stressed mummy and just generally being 2 and testing boundaries.
The two things that have really improved are DD2's sleep (so I'm not a total zombie) and that DD2 doesn't spend the ENTIRE evening screaming anymore. I think DD1 enjoys her sister more now that DD2 laughs and giggles at her.
Just keep repeating 'this will pass". It really will.
I also found doing things with just DD1 very helpful in rebuilding our relationship. It helped me to remember how fabulous she is when not competing with a newborn for my affections.