Most Helpful Guy
Your voice probably just isn't that strong yet. Of course, you'll always have limits to your voice, but with practice you can increase and strengthen your upper range somewhat. When I started singing, I could go from a F2 to a weak G#4, while today I can hit an strong F5 with some effort.
Warming up adequately helps - scales, octave slides, whatever you usually do. If you're going to be doing demanding stuff in your upper register, you should warm up at least 30-45 minutes beforehand. When you sing, engage your diaphragm and carry the tone up from your chest and place it into your head, not letting it get caught in the throat.
There's honestly a lot more I could say, but I'm trying to keep things in basic terms and not write a whole essay, lol. If you aren't already, I recommend finding a vocal coach who can better help you - you won't make nearly as much progress without someone more experienced pointing out areas you could work on. Even if it's just a few lessons to identify your voice type and review the fundamentals of singing, it's worth it.