Speaking from a beginner's perspective, what helps is beginning the swim with an assumption not that you will touch the wall at the end of the 50m, but that you will make a turn there (even if you were to finish just after the turn, but who knows, maybe you will feel ready to give it one more push). It's an advice that I got from my instructor and that has worked great for me (I never expected to make that turn at 75m after I made the one at 50m... but I did make it, and swam on).
Another thing is that it might be that I could push my personal best now, but I don't even want to try it for the time being, because I'd rather improve my technique, which I can feel to be very suboptimal -- I just do repeated 25m, on my arms alone, and on legs alone )and then complete), and measuring the number of strokes/kicks, and measuring and recording the time that I need for that. And I have also found a pool that is only 10m wide, where I do four passes and thus three turns -- and it's the turns that I'm after, to make them as smooth as I can. I expect to try to beat my personal best in maybe three weeks. Until then, it's the 25m for me, and work on the technique until I feel more comfortable/proficient with it. Plus some general cardio work (i.e., jogging and stretching).
I also find that I get cold after 1.5 hour tops. I don't think it makes sense to push on when you feel you're cold, it's better to do something active on the surface, or just exit the pool and get under a warm shower -- you won't beat your best when your kidneys are busy removing water from your body at an increased pace.
Hope this helps, and good luck.