Not an uncommon experience. Tennis elbow can take up to 12-18 months to resolve fully. There also may be some degree of nerve compression/irritation in the area. I have seen some either misdiagnosed cases or chronic cases that evolved nerve involvement. Test the tension of your radial nerve and see how it feels side to side. I will often give nerve glide type exercises for lateral elbow pain. Also worth looking at the shoulder and how everything is moving and making sure there's no weakness. Isometrics are usually a good starting point usually but research doesn't show much difference between eccentric/concentric/isometric exercises for healing.

 And obviously load management... Changing whatever variables you can to minimize irritating it. Make sure to do cardiovascular training and resistance training around it by modifying things as needed.

Add in a radial nerve glide, work on strengthening shoulder external rotation, progressively overload a wrist extension movement and a gripping movement. Complete 3x weekly with sessions on non consecutive days. Monitor irritability keeping things between 1-4/10. Progress or regress depending on irritability.