Cannot maintain erection with girlfriend since a few months ago

Hi everyone, this is my first place to seek 'help'. 27 male, I've been with my girlfriend for almost 2.5 years now and she's my first girlfriend / sexual encounter. Our sexual relationship when we started was great with no trouble whatsoever with maintaining an erection, but since a few months back I've had trouble maintaining an erection in bed when having sex at home. Be it morning, afternoon, or night, after we get each other aroused, my erection would come briefly, but disappear just before or just after putting on condom. Sometimes I last till I penetrate, but I feel myself going soft inside her. The first few times it happens I try to blame work stress (partly true), distractions / loud noises in the early mornings, my weight or lack of exercise recently (slightly overweight).

I had much less problems maintaining an erection while we were on vacation recently, and in fact my hardest erection was while we were having sex in the shower. Was it the excitement of a new environment? I'm not sure, but once we were back home the problem came back.

I'm not sure if I'm still sexually attracted to her. In fact I was excited to get back home to my porn collection after the vacation - I do enjoy masturbating to porn (asian, western, even hentai) which I do so 2-3 times a week. I'm trying to stay away from porn and eating healthier / exercise more to see if it improves my performance in bed.

I do love her very very much and we are planning for marriage, so I'm willing to try anything to see if it improves the situation. Any help would be appreciated.