I am so sorry for your loss- you must be heartbroken, and you need to be really kind to yourself 
DP and I lost our first pregnancy 3 years into our relationship, and I initially did find it so hard to be around my 3 stepchildren (4, 6 & 8 at the time). We held our middle SDD's birthday party at home just 8 days after, and it was so difficult to be my normal bouncy self, leading party games, whilst being surrounded by my SDC/ their friends/ parents & toddlers. I too found it so hard to see my DP being a wonderful father- like a constant reminder of what could have been.
I did become deeply depressed, but being able to talk openly to my DP about my feelings and emotions was essential- we both had to acknowledge and grieve for our loss together, but he also had to be aware of my half irrational/ half completely normal feelings of pain towards being around the SDC. Being honest with him about how I was feeling was the best thing I did, as it meant he tried hard to give me space away from the SDC, or checked if I was ok/ up for joining in with plans with them.
However, time is a great healer, and within a few months I was lucky enough to start feeling that dark cloud lift, and I know that it was actually greatly in part due to my SDC. Their unconditional love, wonder at the small things in life, their constant presence and need for normality and routine, spending time laughing at their antics or moaning at their behaviour helped keep me grounded, forced me to carry on with life, and made me cherish them all the more.
They're dramatic and irritating teenagers now, and they'll never know how much they healed me all those years ago, but I feel so lucky that they unwittingly helped me through that time.
Talk to your partner, tell him exactly how you're feeling, ask him for his help in shielding not only your SDC from your grief, but also to shield you from having to deal with too much daily interaction too soon- in time I'm sure your feelings of pain will lesson, and life with your SDC will become easier and happier again.
It just takes time and talking- make sure you give yourself that 