2011 GMC Sierra 1st gear idol trouble

Hi, my 2011 GMC Sierra started to slip in second
and third gear, but only a couple times per day. I
took it in to my local shop where they told me the
filter was clogged on the tranny and they won't
replace that and change fluid. That was today. He
warned me it might cause harm then good. Now 2
hours later it won't stay in first gear at a stop sign. I
have to give it rpm to engage. Same with reverse,
only engages with a tap on the runs and bumps
into gear.... any help is appreciated.
The auto place wants to do a rebuild I cannot
afford.