The strap-like leaves, sticky sap and the fact it is 7 feet tall in a pot point to the plant possibly being an Oleander (Nerium oleander) (I chose that link since it mentions the sap and underlines its toxicity). See if you can compare your plant with the many pictures of oleanders at various stages of maturity, compare the bark you see, possible dead flowers, fruit capsules and so on and give us some feedback.
Oleanders are heavy feeders and send out strong root systems so it could be your plant has simply outgrown its pot or exhausted the resources. Nothing much interferes with oleander since it is so poisonous despite having an attractive flower. If you have the authority, pull the plant from its pot, if it has formed a clump you could try to divide up the root and repot separate pieces, otherwise propagation is by cuttings. In all cases use disposable gloves and take very great care to avoid contact dermatitis. Otherwise just cut it back and it should re-grow.