There is really no “right” or “wrong” way to masturbate – it’s all about what feels good to you. You are right, though; masturbating through your underwear can be rough on the foreskin and cause drying and irritation. The problem you are experiencing with getting harder erections might simply be that you are worried about it – anxiety and feelings of guilt often cause erectile problems with guys from all over the world. To treat the irritated skin, you might try using a penis health cream (health professionals recommend Man1 Man Oil) that contains moisturizers and nutrients that can soften the skin and help it heal. If you are concerned, you should see a doctor, but what you are going through is normal and common, and things should get back to normal.
Normally it is easier to masturbate without lubricant if your foreskin retracts. Some posts say they are in there 20's and never retracted the foreskin. Some foreskin is thin and almost transparent, while some is thicker and moves more easily. Oil of some kind would help if you have a foreskin that is tight or doesn't retract. You have been doing this through your clothing, depending on how you are situated in your clothing, could effect the time it takes you to ejaculate. While doing it quickly is good when you were younger, if you have the privacy, try and do this without your clothes and can be more relaxed to fantasize, or read or look at pictures. Practice not ejaculating so quickly. Right before you think you are close, stop for enough time till you can go back to masturbating without ejaculating right away. You will become better at controlling your ejaculation. When you eventually start having sex, you will want to last longer than a couple of minutes. Women usually take more arousal, so you may perform, but not be giving her any pleasure. Also when you masturbate try rubbing the area below your balls, this may make you remain erect longer. A dr. can do a blood test to see if your testosterone levels are normal, if not he can give you medication to help.