As auntiejessi says, make sure there’s no physical prblm, altho that doesn’t sound likely. Normally, the start/stop method while masturbating allows you to learn to gauge when you’re getting close & then stop, allow yourself to sort of ‘cool down’, then resume. The squeeze technique can also be helpful. Sometimes premature ejaculation can be related to anxiety - you may just be putting too much pressure on yourself to ‘perform’ & becoming over-anxious could cause it. You should NOT have to be resorting to TriMix injections for this issue.
BTW - I’m a prostate cancer patient & the surgery (07/31/18) damaged the nerves that control erection, ED meds did not help because nerves were not functioning, so I started on TriMix inj’s last Mar. It sounds pretty scary talking about sticking a needle in your penis, but a very fine insulin needle is used & the pain isn’t as bad as you’d think - altho I still don’t really like doing it. The prblm is, if those tissues aren’t regularly flooded with blood (as during a normal erection), they gradually turn to smthg similar to scar tissue, and then even once the nerves have recovered, you still wdn’t be able to achieve erections, so I’m doing this as ‘penile rehab’. That sounds weird but this is the new thinking about helping guys regain their sexual function after prostate surgery. You are instructed how to do the injection & you do it with the Dr. in the office the 1st time to make sure you get the dose & technique right - there are certain very specific areas you need to inject & if you don’t ‘hit’ the right spot, it won’t be successful. Also, getting the correct dose can help avoid the dreaded 4-hr erections, which can damage the penis & in some cases, prevent you from ever achieving erections again - not a pleasant thought, obviously...
First, you want to make sure there isn't anything physically happening here. That's not likely, but it's important to rule that out first.
There are some medications that doctors can prescribe, and some techniques you can try, but if you are literally having orgasms the second you touch, some might be more challenging than others.
When you're alone, there are some things you can do that can help lengthen this. Do you come that fast when you're alone? What happens if you orgasm first and then just spend time touching her? It might take some time to retrain your brain, but intimacy isn't only penetration.
Here are some other tips too -https://www.medicalnewstoday.com/articles/188527.php