There are a lot of factors that can and will effect your ability to penetrate a woman. You bring up the main one how erect you are, but also how well lubricated the woman is. And different positions make penetration more or less of a challenge. And once you achieve penetration that is no guarantee that you will become more erect. Yes there is stimulation once you have penetrated a woman but is it enough, once again lots of factors involved.

And stress is an erection KILLER.

I'm going to suggest a two pronged approach to the issue. Both involve talking to your doctor first, and blue pills.

Viagra is not magic but will help a lot with the erection issue, but that may not be enough if you are overly stressed.

So some Xanax (my generic pills are blue) or other anti anxiety medication to take some of the stress out of you.

And once you get over this hurdle in your life you might find that the medications are not needed. But often a mental barrier gets bigger and bigger the more you think about it, and once you are able to get past it, it won't be as much of an issue.