I usually just think of a generic chair and it goes away.

A couple pieces of advice.

First, don’t refer to girls and women as females when using English. While accurate, it is currently a word tainted in that context by its use among incels and assholes, so it makes it seem like you’re one of those.

Second, at 19, sudden arousal from visual stimulation is fairly common. In other words, it’s normal but annoying. It even happen when you’re 49, though not as often lol

But the way you control yourself is the same way you control anything. You simply choose not to act on it, and wait until the problem goes away. For most guys, the erection will go away in a minute or two unless you’re spending that time thinking about things that keep you erect.

Trust me, you aren’t getting laid often enough just by being around girls/women to merit thinking about it the entire time. So just focus on your breathing, pay extra attention to what people are saying, and wait it out. It will go away eventually for damn near everyone. It has to because our bodies aren’t designed to keep an erection we aren’t using. Now, if there’s enough friction in your pants, it may take longer than otherwise, but it’ll go away eventually.

at 19 not the way in high school but at over 50 I still have to make an effort to be looking at pretty women in the eyes.

The solution is to examine why it makes you uncomfortable. Most likely irrational thoughts are at the root of this. If it’s an involuntary response of your body to external stimuli, then it’s ultimately self-harming and destructive to punish yourself for what you can’t help. Especially when you consider that as humans, we are animals, and animals have the same response.

Try not to lose sleep over it. As @violetraven@lemmy.blahaj.zone said, don’t call attention to it and you’ll be fine. It’s even less weird than getting a boner in the middle of your physics presentation. Here, you at least have an obvious reason for the boner.

It’s normal. Just keep composure and don’t be creepy about it.