Penis size below average ?

You are below average but not small. You can sastisfy a woman. Hopefully no woman would accept you or reject you based on penis size. Hopefully the women you're spending time with see you as more than inches. Almost certainly your penis is still growing, very few men end puberty a month before their 18th birthday, however you are at the end of deveopmental growth, it would be unlikely to see any great change in any of your proportions.

Really you need to worry less about this. Men are measured in acts not in inches and your ability to please a woman is based soley on one organ of the body, your brain. Learn more about sex, worry less about your penis.

When you want to pleasure the deeper parts of a woman's vagina you can use positions that maximize penetration.

I think the woman's size or tightness may actually matter more than the man's size.
A woman can get a kegel exerciser like ezCOME to address the situation.