I hear you. I have two DCs and a full-time job, and it is hard to fit everything in, especially if you lose time you previously had. Doctrine is right re the breathing, I do this, especially at flash points (being late and mess!), and it helps.
But I am also going to agree with 2Rebecca that your DP does have some responsibility, because time you spend with him (probably evenings/weekends) is also time you take away from organising your life. Now that might be time spent in a positive way, but if you are snapping at him, is it just because he is the nearest to you, or because he is putting additional pressures and expectations on you?
He doesn't have responsibility for your children, but if you are spending time together, and your time is squeezed, then maybe there are things he could do to help out? Just a thought. The 'he is busy too and I don't like to ask' excuse is worrying from my own experience, but if he is in it for the long haul, as you say, then it is about your wellbeing too. There is an awful lot of 'I need to organise myself better' in your post, given that you have a boyfriend/partner on the scene who is talking about marriage.
Apologies if I am way off beam here, but I would be looking at ways to share the organisational load before I moved in/tied the knot (speaking from bitter experience).