I just remind myself no discount in lifting and do it full stretch especially last two reps.

I suspect it's your spotter. As a beginner imho you should be lifting without support. Then you'll know for sure how much weight you can actually lift and record your progress from there.

Lets think logically for a second. If you're getting support, you're not really lifting all of that weight, are you?

from my experience,some can some just can't...maybe it's mindset,not sure...
me and my friend had been gym for a month...we start from same weight...
till now,i've increase about 20kg...while he only increase about 10-12.5kg...
i had always force myself,i tell myself sure i can lift heavier than previous 1,while my friend don't believe in himself while i kept on encourage him,but he still can't...
but that's just benchpress...while on military press,we actually hit the same weight from start till now...so anyway,believe in yourself but don't hurt yourself,lift as much as you can...

i think sifu above is trying to said...no support on the 1st few days,so you will know what weight is best for you to start with...not sure,the best just let him answer himself...

what i think is,you MUST teach your supporter,tell them how much you wan them to support as less as possible(cuz sometime they just help you lift all the way while you're not lift anything),tell them support you to perform Full Range Of Motion and balancing as well...i tested this for 2 days,i found my muscle is getting more soreness after teach my supporter a proper way to support...hope this help...

don't worry too much...
i have a suggestion for you...
don't lift too heavy next month,find a suitable weight that suit you no matter how light is it,don't stressed yourself becuz of you can't lift heavier,forget about add weight everytime,only add weight when you think you're ready or the day you feel great...like i said earlier,becuz of i kept on pushing myself,end up i'm using wrong form...you have to deal with it,everyone is different,don't push yourself too much but only believe in yourself,sure you can do it from lighter weight to the heavier weight,it just need time...stressing yourself out trying to lift heavier will only slow down your progress(this is what i experienced)...

about the tiredness,maybe you can try sleep earlier? drink coffee before workout or even try pre-workout supplement to see if it help...

and about the motivation,i actually don't need it anymore,because i'm in love with iron...

check this video by Elliot Hulse, it's been very helpful,hope it help you too...
check his other videos as well...

this is just my experience,i'm just a beginner like you,but i do tons of research,and i do know how being a beginner's feel like,even sifu-sifu here get annoyed by my question already...
but as always,Believe in yourself,but don't stressed,distracted yourself at the same time hope you get what i meant
Happy Lifting
oh,and by the way,i found this forum very helpful...http://forum.bodybuilding.com/...
go there if you can't find any answer here wish you lucks comrade...

Let me ask you. You want to get bigger, are you? Focus when doing your exercise then you can kaplui when resting interval. 
And keep your exercise in full motion even the weight either light or heavy. You will get use on it.

Keyword : Discipline and motivation. It's drive you on.

Let me share what I've done. Initially I can lift about 50kg bench. Now was 90kg with barbell touch on the chest plus 2-3s movement down to chest.