Help with strength and speed?

Hi, I'm currently doing the watersafety swimming for 2 weeks and I'm in rescue 1. I'm doing survival 2 and in that, one of the things I have to do is swim in my clothes for 100metres (could be more) in front crawl. Then I have to take my clothes off (all in the water) and swim another maybe 200 metres (not sure) in front crawl. Then 600 metres in whatever resting stroke (I do breast stroke). I also have to do a timed swim for 8 minutes but I'm not sure of the length I have to do but I have to do 8 duck dives during that.
For the lengths in front crawl, I was struggling in my clothes after around 10/20 metres, but I did it anyway. Then, when I took my clothes off to do front crawl again, I actually was not able to do it. During both times I did maybe 2 front crawl strokes and then breaststroke, front crawl etc. My instructor told me I could just do breast stroke then. How can I get better at this?
How can I improve my timed swim, I didn't get it (I knew I wouldn't, I was going quite slow). I also have to do duck dives, but I can't hold my breath for long enough because I'm so tired with my breathing.
I think I need to breathe better or something, how should I be breathing?

It's just me in my group, I find the breaststroke grand it's just the duck dives and front crawl that gets to me. Please don't tell me I shouldn't be doing it, to drop down because I won't. I was away for the watersafety last year so it's been 2 years since I'm swam really. I'm 14 and I'm not really that fit.

Thanks in advance.