Ballet Strength, Flexibility, and Stamina

Hi everyone! I am a teenager, who hopes to be a professional ballet dancer one day. I am a bit late to start pursuing this goal, (although I have been dancing for six years, en pointe for about three I think) and I'm wondering if anyone has tips for gaining strength, stamina, and flexibility for dance. I feel as though my stamina is all right, although could always use improvement. However my strength and flexibility are not doing so well, and it seems like the more I stretch the worse my flexibility gets, and I'm not sure what I should do for strength. Strength training, or pilates, or something else? Any advice? Thank you!