My guess is that your problem is probably caused by anxiety, lack of confidence, and perhaps some guilt. I am not a great believer in “psychological” causes of impotence, but at 23 years old and in good health (yes?), I think it is the most likely cause in your case.
The good news is that you will get over it.
Using your hand for masturbation is unlikely to cause any lasting problem. Young males do it all the time all over the world without any lasting effect. It is normal and healthy. Don’t worry about that.
Nevertheless, do it less frequently. There is a recovery time during which you are less able to be aroused.
Also, see a local doctor for a health check up. An underlying medical condition like high bllod pressure is a possibility. If your doctor is a good one and you can build up your courage, tell him about your problem. That might be a clue that could point him to a physical problem if there is one.
Eating habits are not likely to have any effect. But if you are smoking, you should stop.
And when you get married, tell your new wife that you are nervous and do not feel confident. (She is probably nervous too.) Hopefully she will be very sympathetic, loving, and supportive. If so, that will help your confidence. If not, you may have the wrong wife and a life of misery as long as you are married.
Best wishes and let us know how you do.