I think it's all about brains, it doesn't seem like you have such serious health problems, so that at your age you have difficulties getting an erection for physiological reasons. Why have you and your wife not had sex for the last three years of your marriage? Perhaps this is the cause of your current failures - you just lost the habit of sex, and now you are not excited by intimacy with a woman as much as you need.
I have a few questions, maybe they will help clarify the situation.
1) Do you have excess weight?
2) Do you take any psychotropic drugs, such as antidepressants?
3) What is the situation with erection during masturbation?
4) Is there a morning erection?
Honestly, at your age such erection problems are unlikely to be caused by physiological problems. As for problems with blood circulation, it is very unlikely. Most likely, in the years that you have not had sex with your wife, you are accustomed to more intense stimulation during masturbation than during normal sexual intercourse. Now it is unusual for you, sensations are not enough, this is the effect. I would advise not to change mistresses, but to try to experience more thrill. A good idea would be vaginal sex, when there is a small vibrator in the vagina - this will be an extra stimulation for you. Or you can try vibrating erectile rings (just don’t think that I am a messenger from a sex shop advertising his products). If this doesn't help, then you can consult an urologist or try taking Levitra or Viagra before sex.
I have a question for you - all during this time when you began to experience problems, did you frequently wake up with morning erections? If so, then you probably do not have a physical problem. Another way to tell - if you are able to achieve & maintain full erections with self-stimulation, that’s another sign that this is not a physical prblm - it would more likely be due to performance anxiety, i.e., you get nervous & worry about your own ‘performance’, which is a very bad word for it & these kinds of thoughts are very destructive. It’s very common & many men have had to deal with it. Just finding ways to be more relaxed, maybe even talking with your partner about it will clear the air & enable you to put less pressure on yourself to ‘perform’ (there’s that terrible word again!)...
This will get dismissed, but I encourage the OP to read up on porn-induced erectile dysfunction.
Viagra will increase blood circulation but not libido( desire for sex) To control your sexual urge( increase or decrease as needed), you may google few exercises like ioAbdominal breathing male deer exercises, kegel exeercises, sat kriya, Breathing exercises are good to control increase erection time and strength of erection. You can search for food which enhances sexual strength. Google foods that enhances sexual strength. Water melon is considered best for this purpose.If you do exercises and eat these foods, you may not need Viagra for erection. This is a long term therapy. Do it regularly. To increase erection time do slow, deep, even (inhalation and exehalation are equal) abdomina breathing. Practice it for 10 minutes twice a day. Use slow breath to sustain erection and do fast breathing to reach orgasm.
Also to add another question in to this. How can my doctor say everything is fine with me and when I take 100mg of Viagra my brain still over rides it and has issues staying hard?