At first, this may seem a little off-topic for this forum, a little too relationshippy... But once you've been around the block as much as I have, you'll see, as I did that this very topic is central to the entire question of relationships with other people, and with sexual/romantic partners especially.

Without boundaries, you will be at the mercy of anyone else you're in a relationship with that matters in any way. People will also judge you on how, when, and how far you set these boundaries. Go too far with this and you'll alienate people and generally be an unpleasant person. Don't go far enough and you invite all sorts of bad behavior out of others. Because people will always treat you the way you allow them to. It's human nature. If you allow someone to take advantage of you, they will unless they have a highly unusual amount of wisdom and maturity.

Also central to the discussion of boundaries is the question of character, both in yourself, and in others. If you don't have the character to enforce your boundaries or just don't have boundaries, you're by definition weak. If you don't have the character to live within your own boundaries or the ones others ask you to work within, you're a hypocrite and a liar. You need to always be looking at a person's character because next to that, nothing else really matters - not their looks/status/wealth, the way they feel about you, how badly they want you, level of commitment. Without character, all of those things are shaky at best, or mirages at worst. We'll revisit this later.

What should your boundaries be? A simple rule is treat others the way you want to be treated. Don't allow other people to do and say things to you that you wouldn't allow yourself to do to someone else. The two catches with this are don't be a hypocrite and don't be a perfectionist. The harsher you judge yourself, the more self-righteous and sanctimonious you will be enforcing your standards on others. This is also how you maintain balance in a relationship - by having sane and clear standards that cut both ways. The key to moderating this is understanding that other people are not the same as you and won't do things the same way you do. There is and must be red lines, but remember that they have to be sane and fair.

When should you enforce your boundaries? Right away when the situation arises. Failing to do this sends the wrong message and will cause you to repress and get resentful. And then one day the bottom will fall out and you'll be overwhelmed with a tidal wave of repressed anger driven more by things that happened in the past, rather than the present. This pattern is unhealthy and almost always fails to resolve the actual problem. Pretty much the only exception to this is when someone is clearly trying to bait or provoke you, then you need to be strategic. Also remember that you cannot negotiate on even terms with anyone that you cannot say "go to hell" to, if necessary.

How far should you go? Depends on the circumstances. This is another reason why you cannot delay enforcing your boundaries - it increases the chances that you'll lose perspective and either go too far or not far enough. Key to this is recognizing whether something is a mistake or miscommunication, or a character flaw. And if it is a character flaw, how deep does it go? This is where we dive into the finer points of a relationship like investment level and trust. The more you trust someone and are invested in them, the more inclined you are to give them the benefit of the doubt. Once again, just human nature. Here's how you check that inclination:

1. How much do they trust you/themselves? Damaged people don't trust themselves and don't trust other people. Without strong character, you cannot trust yourself, even just to live up to promises you make to yourself, much less other people. And also when you don't have strong character, you're quick to assume everyone is the same as you. Oftentimes, I can tell someone isn't trustworthy just because they're suspicious of me for no reason. What people fear and suspect speaks volumes about their internal world and their assumptions about other people.

2. Can they own their shit? People of character know they are not perfect, and know they make mistakes, and know they have flaws. Without this awareness, people cannot improve themselves. Very few people are actually good at this, and even the people who are struggle with this. It's not easy. But it is essential in order to have a hope in hell of having integrity. Generally speaking, the more someone struggles with this, the deeper the problem goes and the lower their chances of successfully confronting it. Which leads us into the next big question...

When should you forgive someone? When they apologize properly. A proper apology has three elements. First is accepting responsibility freely and sincerely, second is doing what they can to undo the damage done, third is taking steps to see that it doesn't happen again. If a person doesn't do this, you can safely assume that they'll do it again. "When people show you who they really are, believe them the first time." Even if they do in fact do these things, it's no guarantee that it won't happen again. Do people in fact change? It's my belief that yes they do, but rarely if ever by design. Most people are just blown on the course their external world, and their formative experiences take them on. Most people just get older, fatter, more set in their ways, and the best of them get a little wiser. Some people however do in fact recognize that they have a say in the matter as well, and because of this, have the capacity to get better, to change, to evolve. But it is not easy. And they must do the heavy lifting, and often on their own, because their fight is truly with themselves. It is possible for people to consciously change (rather than unconsciously), but it is not likely, more the exception than the rule, and it must start with those three steps mentioned above.

I suppose part of the reason why I'm writing this is because this is always something I've struggled with. I like to pride myself on learning from my mistakes, being accountable to myself and others, and having high standards for myself. And it's taken me a long time to learn that most people are not like me, and that I often expect people to be better than they are. So I'm often very slow to write a person off and quick to give people a second chance, assuming that they'll make good on it the way I like to think I would. But this doesn't work. It doesn't work because unfortunately a lot of people are just lost souls, rudderless people, and they don't know it. They don't know how to do the awful, frustrating, and soul-crushing work of trying to be a better person. And if that is the case, the odds of them learning from their mistakes are low. Even if they feel guilty about what they did, even if they miss you and want you back and want to do better. A lot of times they can't forgive themselves because they don't know why they did what they did, or expect you to give them the same treatment in return because it's what they did when they had the opportunity.

So this is my attempt to explain how you balance wanting to trust people and not assume that they're all fucked-up 30-going-on-15 year olds with the necessity of protecting yourself and not wasting your time on people who have proven themselves bad investments.