Help me out in bed. A whole slew of problems... Starting with not holding out.

Throwaway for obvious reasons because I haven't talked to anyone about this other than my SO a bit.

My SO [29F] and I [25M] have been together nearly 4 years and lived together almost 3. We've had a great relationship in the past but its been rocky lately and I think a lot of it is sex related.

When we first started dating, once we started having sex (I still feel we moved a little fast as she is a highly educated woman with incredible morals but something between us clicked and we started having sex 10 days after we met) anyways, we were having sex daily, a lot of the time, multiple times a day. It was great. We tried different positions, loud, passionate, sweaty raw sex! Since then, our sex life has taken a HUGE dive for the worst...

We hardly have sex anymore and when we do I know she doesn't enjoy it. She always would tell me she's fine but I knew something wasn't. She's the type of SO that doesn't want to hurt me so I have a feeling she was hiding some feelings. I tried to step my game up but I have a serious problem... I can't hold out for literally 15 seconds. I hate it. I get in an angry mood because I'm so pissed off at myself but I have no idea how to fix it. The premature mess started just under a year after we started dating and to make up for it, I go down on her. Every time we have intimate time, I go down on her until she gets off. I'm not the selfish type to where I don't care, I literally get her off orally every time to make up for my measly 3 pumps. :( so I feel I used that as a balance for my lack of performance.

I feel it all began after the honeymoon phase started wearing off and I remember a few times doing her from behind, getting all sweaty and her body language would tell me she wasn't enjoying it so I would focus on busting. Another thing that might help explain the problem on my end is that she use to tell me she loved feeling me cum inside her. So I use to fuck her thinking about cumming because she loved how it felt.

We talked in a non sexual environment the other night and I told her just to tell me the truth and I won't get upset. She admitted that she wants me to last longer and that I'm less attractive when I put on weight. I use to be 270 but lost about 60 of it and was eating healthy going to the gym. Well, I've since got a full time job and go to school Monday-Friday. I leave before the sun goes up and don't get home til after the sun has long set. And my weekends are consistent of working and school work so I can focus in being as wealthy as I was so my weight has been coming back, not 270, but not 215.

Some more background is that I've had this undesirable trait where I use to be in a pissy mood when I dont get sex and I would hardly talk. After realizing what an amazing woman I have, I decided about a year ago that that shits gotta change or I'm gonna lose her so I've been working on co trolling my dramatic ways. Just this morning I woke up horny started rubbing her and he expressed that she wasn't in the mood because she had to get up and on with the day. My initial reaction was that I was upset but stopped it. Didn't get butthurt and got over it. It was refreshing that I could tell my labido, not now mother fucker.

So basically I've turned this into more of an r/relationship post so I'll get down to my initial reason for posting, can someone help me last longer? Please be honest with me but don't insult me please.

TLDR Not happy with my performance, SO finally expressed she isn't either.