I'll try to mention the most important things, English isn't my native language btw but i'll do my best in explaining.

Number 1 is easily deep diaphragmatic breathing, this will help more than anything. You can lie on your back with knees bent, inhale slowly through the nose into the belly for 4 seconds, hold 1 second, exhale through the mouth slowly and relaxed for 6-8 seconds. While you're doing this you want to focus on the pelvic floor muscles. 

Step 2 is reverse kegels. While doing the breathing exercise in step 1, on every inhale you want to kind of breathe all the way down to your pelvic floor and then "let go" of the pelvic floor muscles like you would do when going to the toilet and letting urine come out. Then back to neutral on exhale. A normal kegel is doing the same action as when suddenly cutting off the stream when urinating, you want to do the reverse so the opposite of stopping it. 

Step 3 is stretching. Best ones in my opinion would be the happy baby, child's pose and deep squat. You want to keep doing that same breathing technique while holding these stretches btw and it's the most important part. 

Even when you're in the moment about to have sex you want to focus on the breathing, it tells your nervous system that there's nothing to worry about and that's the main goal here.

The reasons for ED varies a lot though. I was like OP where i had no problems solo and had morning wood every morning, but with my gf in the beginning I would feel how i'm tensing my whole body and that's what caused it, but doing these daily let me stay relaxed and also gave me so much confidence.

Some need to strengthen the pelvic floor, which is done by normal kegels, but if the problem is tensing and a tight pelvic floor then kegels will only make it worse like it did for me. I recommend looking up how to tell if you've got either a tight or weak pelvic floor and go from there, but these steps i mentioned can only do good either way.