Not a relationship expert but here is what I believe in:
1. Women are filled with emotions while men can't relate. We also have emotions but we are not as emotional as they are. Due to that, it makes them to overhink more, dive deep into details and come up with all types of scenarios that didn't happen, haven't happened and won't happen. Once again - they are emotional and love to overthink.
2. If you've decided to get into a relationship, calm your hormones down a bit and think straight - do you need it right now? Is it THAT necessary at the moment? There are more females on planet Earth than males, what makes you think that this girl is the one? Seriously, think about it.
3. If you already got in a relationship with her, makes sure to set your priorities, standards straight and stand your ground. By this, I mean - don't let her get in your head too often, don't put her in a cage by not letting her to do whatever she feels like and eventually, she won't do the same to you. Let her hang out with her friends and those silly little boys that she is being friends with. If the girl actually likes/loves you, she won't be affected by any type of flirting or whatsoever. Put the bitch to test, simple as that. If she does get affected, then she wasn't even seeking for a serious relationship, she just wanted to have fun and see if it's something she needed at that time.
Basically, don't be a bitch yourself, stay organized and work hard on a daily bases so you become a better version of yourself by each day otherwise you'll fall for this bullshit, break down, start drinking and whatever the fuck these modern-day snowflake type of men do when a girl breaks their heart. Step your mentality game up.