This has been an ongoing issue within the past 2-3 months where the tip of my penis has become acutely more sensitive than it used to be. I recall this problem beginning in the near end-relationship with my previous ex-girlfriend who I was sexually active with (roughly at least once a week, sometimes fucking at least 5 times in a weekend). On average I would last around 20 minutes and at most up until 40 minutes or so and I was satisfied. But near the end of our relationship at least I noticed my penis had become more sensitive and I wasn't lasting as long, and since then up until now, there's been a notable difference in how sensitive it's become. I've periodically suffered from depression and used to take 50 mg of sertraline (anti-depressant) a day, which did not affect my sexual performance; however, I recall after I stopped taking them around the time this problem started so I was wondering if maybe this had anything to do with why I've been experiencing this? Possibly low serotonin levels? Another possibility I considered was that I conditioned myself into "going rounds" so to speak, rather than being able to last long from having too much sex?
I'm not the healthiest in the sense that most of my life I haven't been the most physically active and my cardio isn't too good right now, but recently I've been going to the gym every day/every other day for the past month. I also used to smoke marijuana occasionally but even then I didn't notice any problems with my body and for the past couple of months I've been sober.
Some additional information is that I was circumcised from birth, currently, I have no trouble getting myself hard and that I have also been limiting myself to masturbating only once every 3 days or so and am not an avid porn user.
I'm 17, 6'1 at 162lbs. I'm aware my body is still growing but it's still a concern of mine knowing some part of me isn't working as it used to without a definitive answer/reason. Since I don't have a doctor available to me locally at the moment and I've ignored seeking help for this issue until now, I would deeply appreciate it if someone could provide some guidance and clarity towards solving/understanding this problem of mine. If there is any other information that I have left out which is needed I would be more than happy to let you know.