I have been with my boyfriend for soon three years and we live together but he always betrays my trust. I recently found out that he has kept underwear photos of his ex and kept them intentionally. He also took underwear photos of one of my friends while she was sleeping...
He also contacted his ex behind my back without telling me.


I know that this is a lot of things that he has done but our relationship is also great and except for when he does something stupid we are very happy and loving towards each other. He is the person I trust most (ironically) and he has also helped me to overcome my depression that I have had since childhood. So even with his mistakes he is a great guy and we both benefit a lot from our relationship and love each other deeply.
Hence I am feeling very torn about what to do as I know that he has done things that are completely unacceptable in a relationship but this relationship is still very meningful and we are a very compatible couple.

Any advice on what I should do, break up or accept his flaws?

I'm confused, so I guess you are too. You start by saying twice that he 'always betrays my trust,' yet you trust him the most, and you are very happy with him.
I don't see where 'always' comes in.
Thinking back over my 68 years, I can remember times when I contacted an ex without telling my present boyfriend/husband. The NATURE of that contact is what matters, not the contact itself.
I am friends with all my exes.
As for photos of women in underwear, that can run from mildly unfair to serious, depending again on the intent.
What I do find truly wrong is taking pictures of someone without their permission, specifically the woman sleeping in her underwear.

Find a time when you can talk calmly about each part of this, tell him how you feel without accusations and anger, and tell him that it hurts to know he keeps pictures of ANYONE in underwear. What if he breaks up with you and keeps similar pictures of you? That happens all the time when people break up, and the pics are used for revenge.

Again -- this doesn't mean that he is a horrible untrustworthy person.

You are just a live in girlfriend, and too dependent on this fellow (Or the relationship) I might add, and are to afraid to rock the boat by expressing yourself honestly. I can see being grateful for the positive influence he brings to your life, but NOT depending on him to cure your flaws. You cannot cure his either, nor is he the only one who does stupid stuff in this relationship. NO WAY! Maybe he just hasn't told you of your flaws, or handles them differently.

So my advice is to learn to calmly express your displeasures, and work on your own trust issues so you can see what you need to do for yourself and this relationship. You cannot ignore that he keeps these photos and is willing to sneak and take photos of others.

What would happen if you got rid of these pictures, and read him the riot act about taking secret personal pictures of your friend? That's what a healthy person in a healthy relationship would do, and not just sit feeling bad about the unacceptable crossing of boundaries of good behavior.

Is it really stupid if you allow it to continue? This isn't about trust, it's about you not expressing yourself. Or are you afraid to TRUST yourself? You don't get to complain about his stupid stuff if you allow it, and don't deal with it.

Without trust, you have nothing. Nothing.

The very most basic denominator in any serious relationship is trust. Everything flows from there. When suspicious behavior becomes fact, as it has in your case, it is time to make changes.

Abused women will say that other than the beatings, he's a great guy.

Girlfriends of alcoholics will say, other than the drinking, he's a great guy.

Other than his panty pictures and contacting his ex on the sly, he's a great guy.

Patterns in bad behavior, tend to repeat themselves, rather than be a one shot slip, that would likely forgive in an otherwise solid relationship. Patterns that keep popping up like a bad rash, need some corrective measures.

Try talking to him. Give him notice you have some concerns that need to be addressed. Then tell him. Talk, and then listen. Really listen. If he is willing to address the issues, and learn to talk to you before acting on them- again- then you have a chance at saving the relationship.

If he is full of excuses, and turns it around to you being paranoid, and accusatory without fact, and in other words, YOUR problem, then you know that things will not likely change.

And in my experience, people (not just men) tend to downplay their behavior. "I only had three drinks- you're imagining things" or "you make me do it" or "It means nothing that I contact my ex and take panty pictures". At best, you will get a little bit of the truth, "ok. ok. I drank a quart of hooch" or, "the next time you make me angry, I'll punch the wall", or "I had good reasons to contact my ex in secret, and I only took panty pictures once"... oy.