Not a sifu here , but here's my experience :

1) 3 years now , however first 2 years were at home (restricted to 35kg max only) , the other 1 year and counting is in the gym , since I've signed up for my membership last March.

2) So far I've injured my lower back twice , the first injury wasn't severe (from over excessive deadlifting during the first 2 years at home when first starting working out) . The second one was due to Squats (and boy it was a horrid incident) , I jumped straight into my max lift on my last set , right after a one month break from the gym due to my mock exam 

Hope you recover soon bro. Check out the stickies here on the proper forms of deadlifting 

I see ... Well it took me almost 2 weeks when i injured my lower back on the 2nd time. Perhaps u could video record ur form for us and the sifus can help identify any mistake there is.

no need to feel embarrassed bro. we all gotta start somewhere. Better post and correct rather than continuing with bad form. Besides,  why would we make fun you, we're here to help

btw, check out the stickies here for the proper forms on deadlifting .

Tighten your core and push it off the ground steadily.
Don't use explosive strength especially with rounded back. That's a shortcut to snap city. Took me 2-3 weeks to recover.

Last time when i snap my shit it tooks me several months before i start weighlifting. (Yup stopped weightlifting, u heard me rite).

Then i watched elliot hulse video about why u snap ur back? Then i found the method to deadlift properly.

Before you start to lift the bar there are two muscle group should play a part, abs & legs. When u lift start with using abs muscle (Tighten of course) then use ur leg to push upward, then at last u use ur back to be straight.

Dont progress so fast (in-terms of weight) for this three dangerous exercise Deadlift, Squat and bench press.

Remark: Always make sure ur core is good and stable.

we're about the same weight, and deadlifting the same weight too

for the weight that sometimes feels so heavy while other time feels easy to handle, there's alot of factor that can influence it, eg. last night slept well or not, foods before workout, your mind condition and so on, we can just call it a bad day and dont force yourself doing heavier one, attempt the heavy lift at next workout

i love do 1 rep for deadlift too, i can spend a whole hour doing deadlift multiple sets of 1 rep to see how far i can go    but yeah, need to really emphasize on form when deadlifting near your max

Then i would say it is your mind condition since you got experience in deadlift.

Try to visualize first before you start lifting, i always do that and i dunno where i read this before.

This will help in mind muscle connection to prevent injury and boost self-confidence for heavy lifting.