Truth to be told, unless you are at beginner level, you will lose some size. If you are 220+ , the best thing you can do is to try to protect it with that low levels of hormones..
During my last pct , my test was mid 200 about 7-10 days after cycle and I was literally killing myself to get one extra rep.. And of course in the first week of crash, I lost 4-5 reps on most exercises.. I had very high protein and cal intake and gave everything I could to my training and kept almost all size.. Now my test is back to what it was normally (800-1200) and I have no problem adding weight to the bar even with much lower protein and calorie intake(and higher intensity training..)
I know that psychology very well.. But definitely you are not going to lose ALL muscle.. Keep diet and training in check and you will keep most of your gains (and almost all of it if your cycle was anabolics only).. You are going to feel like crap , have depression-like effects but be tough and it is gonna pass..
And also , 300 is very low level for a 30 year old.. The lab ranges,, yeah the low limit says 230 something but it is for 70 80 year olds.. I felt like shit at that level.. So go to an endo and do what he says , it nothing else works, go to HRT.. That way, you can build lots of muscle even after using AAS..
Also use peptides, creatine , test increasers like unleashed during your therapy.. It helped me a lot.. I believe there was no way I could have kept my size with that low test levels during that week or two..