Like most who’ve been training for any length of time, I’ve also experienced my share of pulls and strains. Most of these were minor, healed quickly and resulted in only a short interruption in my training. A (thankfully) small number were more serious however, and took time, careful planning and hard work to rehabilitate.
As a career trainer, I’ve also worked with numerous clients who’ve come to me looking for help rebuilding after a serious injury; and I’ve seen the same type of situation you’re describing with regard to squats when someone has experienced a back injury, time and time again.
So my first suggestion to you is to cut yourself a break; at the best of times stepping back and dropping into the hole with two to three times your bodyweight in pig-iron across your traps can be scary. Normally the confidence we have developed in our ability (with time and experience at performing the lift) enables us to turn the fear involved in performing a heavy squat, and use the adrenalin rush that comes with it to our advantage; but your current situation is by definition, abnormal for you. You have experienced a sudden and dramatic change in your situation, having recently suffered an injury, and you have reason to be unsure how much your body is ready to take. Properly managed, the fear you have of re-injuring yourself can be a valuable tool in your rehabilitation though. It can be used to prevent you from training recklessly when caution is appropriate.
That said however, you have to know that it isn’t reasonable to think that six weeks of downtime could have degraded your physical capacity to produce force to the point where you are going to be incapable of handling less than a third of your previous personal best. You know you have more in the tank than 135 (otherwise you wouldn’t be asking the question you are). The question is how much you are ready to handle without pushing too far too fast, and how to make sure you don’t cross the line and do something stupid that will create a further setback.
Experience (both personal and professional) has taught me that the best way to deal with this type of situation is to be systematic about it. By that I mean sit down and plan your recovery training. Set out a reasonable length of time based on sensible increments in training load, and plan what you are going to do during each training session. Begin as you have, with conservative loads and higher repetitions per set. Over the course of time increase the loads and reduce repetitions to ease yourself back into heavy squatting. Use the time to work on your technique and supplemental exercises, and become a student of squatting. If you play this right, take your time, and focus on perfecting your technique as you rebuild your confidence, you could very well come out it a far more capable squatter than before.