I don’t know what to do: I can’t control volume with very high notes — I always have to sing very loudly. If I try to soften it, the note just goes flat. The volume isn’t really the issue and I’m not going out of tune when I sing loudly; it’s just I physically get tired — I feel my whole body going weak after the first chorus. After that I literally don’t have enough air to go on.
It would be wonderful for you to invest in a few lessons with a good teacher. Muscular strength develops over time. Just as we do only a few repetitions of weight lifting exercises and then take a break before doing more, we can develop our strength as a singer by working for short periods of time more frequently. Instead of practicing for an hour, try practicing 10 minutes twice in the morning, twice in the afternoon, and twice in the evening. Practice high notes for only 3 minutes in each session. If you work within your present capability you will teach yourself that singing high notes is easy and fun! We all find it easiest to sing high notes loudly at first because it takes more strength to sing them softly. Once you can do the loud ones without stress, you will naturally gain more flexibility and choice with volume.