problem in having strong and lasting erection

I am 35 yr old male, married for past 3 years.
My problems now are:
a)less libido,say once in 13 days
b)even when libido is there,my penis do become straight but during intercourse,when I want to enter it to the vagina,it simply becomes soft and normal.Thus,I fail to enter.
c)so,during libido,I do have ejaculation but that's it.
d)Tried allopathy for some time. No significant improvement. For example, for one month I took a tab called restore. No such gain. When,I took Viagra (50mg)one night, I didnot have a natural libido then. so,no erection. Later on,I came to know,viagra keeps penis straight but I need to have libido then.
e)I am a software engineer,9 hours on pc,5 days a week.
f)I am 5feet 9 inches tall and weigh 100 kgs.so, bulky and obese.
g)as physical exercises, i take 45 mins leisure walk 5 days a week and started going to VLCC slimming centre where they make me go through passive exercises through various machines.