How do I find contentment without ceasing to strive?

A little over a week ago I started working out again after a month off, at a slightly less intense pace. It was going great until I had my legs knocked from under me by fatigue. So I haven't worked out since Tuesday, and won't really get a chance to till this Tuesday because we are going out of town. 

Because of all of this, I'm feeling like I am never ever gonna get the rest of this weight off. Part of me is really frustrated and depressed that it's been such slow going, and the other part is like "Hey, go easier on yourself, you're still nursing, and your body is hanging on to that weight to make sure you can nourish Lexi!" And I get that-I really do. I just don't want to give up on the things I want for myself just because I'm a mom. I am perfectly ok with knowing that it could be another 3-5 years before I have abs again (since we are about to start work on baby 2, and hope there will be a baby 3 down the road about the same distance), but I'm not ok with giving up on or stopping work toward that long term goal. Slow and steady is fine with me, especially as we start TTC again, but I don't want to stop progressing or lose ground on my health or fitness even while I'm pregnant. I'm going to take Lexi on lots of walks, and carve out Yoga time, even when I can't spin or Zumba anymore. I must do this for me.