Erectile Dysfunction. Worried on this.

I got married two weeks ago.I am able to perform well for the first 4 days in bed. But after that my penis gets erected during foreplay , but before intercourse it lost its thickness..it is erect but not thick enough to have a intercourse. I am able to masturbate successfully. But last four days I could have sex..It's not that I don't have erection..its that I couldn't maintain it. Before marriage, I do have some private time with my lover(my wife now), except intercourse we indulge in foreplay .Where I will have full erection and it is maintained for more than 2 hours during the entire foreplay. Also after marriage if I wear a condom , my penis loss it's erection immediately and its very hard to regain the erection. Also only for the first 3 days of marriage I can hold for more than 10 to 15 minutes during sex, after 3 days I can't even hold for 5 min. I ejaculated quickly Please help me immediately as I am more worried on this.