Unhappy Bonsai

Hello. I've had my precious Chinese Elm for 2-years and it was doing great until 3-months ago. The tips of the leaves turn yellow, then brown and then fall off, all within a few days. I've left it, misted it, moved it but its condition got worse. A friend told me it might have spider mite although there was no sign. I sprayed it with Provanto Ultimate Bug Killer and it seemed better during treatment but the diseased leaves return as soon as I stop (after 2 nights the first time, 5 nights the second). Photos attached. Any advice would be very much appreciated. Thank you

I guess you keep it indoors? Indoors kills elms. As soon as risk of frost is over move it outside. It will need slow transition from shade to full sun. Once outside in full sun just leave it there, never taking it indoors again. If you have severe frost, say below -10 for weeks some protection is beneficial, shed, cold frame or unheated greenhouse. Never living toom conditions.

I do keep it indoors. I've just moved it outside in a sheltered spot, we'll try that. Thanks for replying and for your good advice.

put it in the sun and water it well