Reading your post took me straight back to my childhood. I was in a similar position to your children. My schedule was 1,1,1,1,3. It wasn't very conducive to structure, even though my parents lived within a mile of each other and my school.
Needless to say, I completely empathize with you and what you're going through. Custody issues are so messy and complicated, fraught with frustration and other emotions. Two things about your post jumped out at me, in regards to the ex. First, the friction there seems to be causing a lot of pain. Are you at the point where you can have meaningful communication? If so, set him down and explain to him that it is not appropriate for him to castigate you in front of the kids. Make a pact not to do it. It hurts everyone, and no one wins. Second, it was inconsiderate of him to sign her up without agreeing to a compromise with you.
You sounded really bummed about having to watch your kids play sports rather than interact with them, but I can tell you that those memories and that support you gave is extremely valuable. Seeing Mom sitting on the bleachers cheering her heart out is a privilege not every kid gets to have. Your daughter is fortunate to have the opportunity to have you present. I know it's not anywhere near your ideal way to spend time with your daughter, but she'll always be glad you were there.
As far as this season goes, I'd suggest that you sit down with your daughter and have a chat with her. Tell her how hugely important your relationship with her is to you. Tell her how much you love her and support her, no matter what, first, and always. Make sure that she knows the issue isn't about softball or scouts, it's about time together. There is no substitute for time spent together. Try to structure some time together during your week. Ask her how she would like to spend time with you. Come up with some ideas together. However, if she wants to play ball, then tell her you will be there to cheer her on.
It's most important for you to show love the way she understands it. At this point, the way she may feel most loved by you is by having you sit in the bleachers cheering for her. If so, that's where you need to be.
Don't worry about how fast time is passing. If you communicate to your kids your love for them in a way that they understand, your foundation is solid, and the relationship will continue to grow. There's a lot of fun and exciting milestones ahead, even through those teen years. They will need you more than ever; they just won't verbalize it.
Hang in there! Hope it works out well for you.