Swim Pacing and Fatigue?

As you swim more, the rest intervals will come down.  You might consider focusing on shorter length intervals (50s and 100s).

Slowing down is basically psychological.  You have to convince yourself that it is OK to go at a pace that feel ridiculously slow.  Make it a challenge.  You're swimming around 1:50/100.  Challenge yourself to swim 100 in 2:00.  If you come in under 2:00, you failed.  Try again.

It sounds like you need to work on your balance in the water.  There are drills that can help.  It makes a huge difference.

Your splits from the 4 x 150 work out to an average pace of ~ 1:48/100y.  That pace indicates you have technique issues that need to be sorted out.  Those technique issues are causing you to become anaerobic while you swim which directly leads to the fatigue you are experiencing.  This ISN'T about pacing at this point.  If you slow down in an effort to delay "blowing up" you'll actually make things worse because your balance in the water will be even worse - your legs will drop more than they likely already are, causing you to work harder and forcing you to become anaerobic even more quickly.

The best thing you can do for your swimming at this point is arrange a series of lessons with a qualified coach/instructor.  As you improve your technique, I'd suggest shorter intervals with sufficient rest that you can maintain proper technique.  There is nothing wrong with doing 50's or even 25's.  As you build your swim fitness you can lengthen the intervals.

Using your times from the 4 x 150 as a guide, you might consider starting out with something like 10 x 50 @ 1:15 send-off.  That will get you started working on the fitness part.  More importantly however is improving your technique.  Good technique and swim fitness are two sides of the same coin.  You can't have one without the other and they must be developed simultaneously.  The quickest easiest way to do that is through lessons.

Hope that helps.

If your legs are dropping AT ANY POINT or at any speed, you likely have similar issues to the OP - you have technique issues that need to be sorted out.  See my comments to the OP.