It depends what exercises you are doing. I wouldnt reccommend doing all muscle groups on same weight, adjust accordingly. As the 15 may be to light /heavy for different muscles.
But its great that you have went heavier thats the key.
10-12reps is great once you can do more you need to go heavier or you will step in to endurance training not strength.
Youll be amazed at what you can manage.
Ive went from really light womanly weights to quite heavy in 4months and seeing FAB results , still have fat to lose but my muscles are shaping nicely, i actually have a bicep
My gym work in kgs
heres a guide to what i lift now and at first i was on the lowest.
Biceps 10kg =22lbs
chest 12.5kg = 27.5lbs
Lower back 20kg = 44lbs
upper back 35lbs
thats just a rounh guide and it hasnt took me long to build my strength i started on 4kg dumbells 8.8lbs
Its really worth the effort..........keep at it and good luck xx