Can you shrink a 50/50 Polyester/cotton shirt if it's already been washed and dried (in dryer)?

I have a shirt that is a little too low-cut on me (a v-neck). It is also a bit big in the sides. The sleeves are about just right, but they can shrink a bit and be ok. I already washed it and dried it on high, but it did nothing! Would I have to wash it in hot water? It will fade. I think I washed on warm, rinsed cold. Can it still shrink some if I put it in the dyer again? Would it look bad with a cami underneath? It is light blue with some white and flowers on it. I guess I should return it because they had one small left, but I dunno if they'll take it if they found out I washed it.