Improve Stamina

Swimming is about technique more than strength or aerobic condition.  If you can hire a coach or take some lessons where a knowledgeable person can watch you and offer corrections, you'll make great progress.   (Far more than trying to incorporate vague internet feedback.)  

I was a lifeguard and swim instructor a long time ago.  It's common for people to be uncomfortable about swimming and they are tense and try very hard to get out of that discomfort.  Because we cannot breathe under water, it's natural to "hold your breath" which creates tension.  Your muscles are fighting one another which wears you out quickly.  Additionally, there is a tendency to lift the head to get a breath and again, because of anxiety/tension, this leads to a "blast-and-gasp" approach to breathing (blast out the air you're holding in, then gasp for as much air as possible while your face is out of the water.)  Of course, with the head out of the water, the legs sink down.  (Picture a canoe with two people in it.  If the people are in the front and rear, the canoe sits flat on the water and is easy to paddle.  But if both people are in the back, the front is high out of the water, the rear end sunk very low, and the canoe will be hard to move through the water.  That's YOU with your head out and feet sinking.)  

My general advice is:  R-e-l-a-x and s-l-o-w d-o-w-n.  Practice gliding by putting your face in the water and pushing off from the side/end (I'd call this "Superman").  Make changes to hand position and head position and note which combinations allow you to glide farther.  Glide on your side and also on your back.  Practice starting like Superman and finishing on your back or side.  (Remember to practice gliding on EACH side.)  

As you glide, you can work on breathing.  While your face is in the water, hum gently.  The air coming out of your nose will help keep water out of it.  Starting with Superman, glide and hum, and when you want air, pull with one arm to glide on your side.  Key is to R-o-t-a-t-e rather than lift your head.  

After you can transition from Superman to side, you need to transition back to Superman.  For simplicity, assume you're on your left, which means your left arm is extended over your head, left ear is in the water, right arm along your body, and you're facing the right side wall of the pool.  You'll need to move your right arm from along your side to out in front next to your left arm and your face back in the water.  Since you're on your left side, your right arm will naturally be out of the water as you try to bring it beside your left.  

If you think about going slowly from Superman to side and back to Superman, you'll recognize this is about half the swim stroke.  If that time on your side is nice and long, you won't have trouble getting enough air to go back to Superman.  Once you're confident you can get enough air, you'll relax more.  

Think of a tri-suit as a swimsuit with a small pad on the butt to make riding your bike more comfortable.  It won't help with bouyancy (but the above will).  If it did help with bouyancy, it'll simply mask the problems and delay your ultimate progress.  

What are you doing for swim training right now? 

For super sprint to sprint distance your swim practice yardage should be at least 1200-1500 yards/meters.  ie. 200-300 warm up (swim/drills/kicking), 40x25, 20x50 or 10x100 or some combination thereof, 200 cool down. 

As you get closer to your race, you'll want to start prepping for the distance with longer intervals on short rest.  I don't think your longer intervals need to be more than 200-400, even for HIM/IM swims as long as you keep your rest intervals short.  For super sprint and sprint distances, 100's on short rest are fine...you can add some 200's if you're worried about pacing.

It's not a bad idea to practicing sighting in some of your intervals.  It takes some additional fitness to sight and swim.