Trying To Lose Weight. Having Erectile Dysfunction, Early Ejaculation, Tiredness During Intercourse. Solution?

Hello Doctor, I am 32 years old. I got married 2 years ago. I had gained lot of weight after marriage, but in last 4-5 months I have reduced around 45 pounds by diet and exercise. I am still trying to reduce my weight (i.e I am eating less calories and having exercise at least 2-3 times in week). My current weight is 180 lb (82 kg) and height is 5 7 . We are planning for baby. We are trying intercourse since last 3 months during ovulation timeframe. Whenever we try to intercourse, my penis loose strength immediately. When I lay down on my back, my penis erects for long time without any issue. But when I sit on my knee for intercourse, I get tired quickly and loose strength during intercourse even before going inside vagina. I am also facing early ejaculation during intercourse. I didn t face this problem during initial few weeks after marriage. But we really didn t try intercourse in last 1.5 years. But I used to do lot of masturbating during last 1.5 yrs (twice a week on an average) Can you please suggest what could be wrong here? How to keep erection of penis for long time during intercourse? Is there any specific sex position that we should try for this? Thanks for your help.