Just don't do sit ups! That can permanently damage your tummy muscles if it is the DRAM thing (diastasis rectus), which it sounds like it is. Everyone gets separated tummy muscles after having a bub, but the extent can vary. I think two fingers width is normal adn anything beyond that requires a physio. I'm suprised your midwives/mchn didn't check yours just after the birth? Though maybe they did, you've prob forgotton..I see your bub is 9 months! It's important to work on getting your muscles back together, spech if you're planning another baby. I never worked on mine properly after bub #2 and as a result had far worse back pain during my third pregnancy.