Can anyone guide me in the direction of resources for learning telepathy and sharpening that skill? Any advice? I would love to be even more receptive and a better sender of messages.

There are many ways to open to the sensitivities within each of us.  What messages are you intersted in sending?  Why telepathy?

This is often opened between two compatible souls without any practices as such.  You might want to figure out your own purposes.

Because I have always had a strong telepathic connection with my now ex, and I would love to send him messages of love and whatnot..Now that communication between us by other means (talking, writing) has been shut down completely.

There are many ways to do telepathy, my preferred method is usually a psi ball programmed to send a message, and also linking.

To do a telepathic link, I visualize a dense pipe/tunnel connecting from my crown chakra to the reciepient's crown chakra. Solidify it, and you'l be able to send messages with your head already. Take note some advanced psi manipulators may be able to block this telepathic link at the crown chakra, in which causes a slight headache surrounding the brain.

For a psi ball, visualize energy coming out from your hands and form it into a ball, fill its inside, shell the outside (to prevent the energy to dissipate or leak) and then program/tell the psi ball to go to (reciepient, you can visualize them or say their name) and include a message. You might want to repeat your message a few times in case the other isn't able to pick up the first mssage. eg: "buy pie for me plz? Buy pie for me plz? Get me pie. Buy pie for me" Also, the psi ball is really effective if you think this way - energy travels in a straight line, and the distance between an individual to the other doesn't matter because the energy you give off can be recieved anyway.

Advice: open up your crown, thrid eye, and throat chakra. It'll be more effective. But it won't really matter if you have a balanced chakra system

This is a course of study on mediumship.  Telepathy is not a part of that practice.  You could take this topic to another board for more input on this skill.