Starting to swim again

I was on a summer swim team for a few years in middle school, but quit when I was 13 due to how toxic the team environment was (coach forced us to swim during a thunderstorm as the rest of the pool was evacuating). It's been about 6 years since then, and I've developed moderate asthma. I recently started swimming again and I've completely lost my form, and my breathing sucks. Are there any exercises that can help with lung capacity? I can practice my form, but only if I can actually breathe lol