I am ready to give up again and resign myself to a life of being a fat slob[/quote]
Let me just address this first… this is not the time or place for a pity party. Exercise is hard, eating right is hard, results are worth it. Getting momentum going in the beginning can be tough, but once you figure it out and get your fitness lifestyle in place, it’s not any kind of major hurdle. Lots of badass ladies on this site have gotten into killer shape after having kids. Look around for their training logs to get some motivation/inspiration.
As a parent, you have a responsibility to be some kind of good example for your kid. At the very, very, very, very least, caring for a baby is tons harder if you’re a fat slob and a smidge easier if you’re in shape - everything from carrying kiddo around all day to running and playing with them at the park instead of sitting on the bench watching them.
If you really are using great technique for every rep of every movement, I’d examine your intensity, frequency, and overall program design. A poorly-designed routine could definitely lead to injuries.
What does your week look like? The days, exercises, sets, and reps. The more specific, the better advice we can offer. Tabata, by itself, is a brutal workout. I don’t think it’s appropriate for someone coming from where you’re coming from - a deconditioned beginner trying to get back into a consistent routine. Plain old cardio intervals can do the trick just fine.
There are tons of good programs here on the site that can help get you back on track, depending on your available time and equipment. If you have a gym membership, perfect. Lots of great routines are just 3 or 4 days of training. If you’re training at home, what equipment do you have?
What was the injury? A minor strain that’s clearing up already or something more serious like a disc issue? Also, for reference, what’s your current height and weight and what kind of shape are you in, bodyfat-wise (lots to lose/pudgy, skinny with no muscle tone, etc.)?
Feeling tired the day after a serious Tabata workout means you probably did the workout right. But it could also be that your total calories and/or carbs are too low.
What, exactly, did you eat yesterday?
Is there a chance you’re having a reaction to the shakes? If the only change you made was to add them, that’d be my first guess. Second guess is that the shakes are added a bunch of extra calories to your daily intake that wasn’t adjusted for elsewhere in the diet.