I don't understand boundaries and feel flooded with guilt

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I grew up without any sense of setting my own boundaries. I don’t know how to say no and not feel guilty. This has led to things like attending stuff I don’t want to do and in more extreme moments, agreeing to sex acts that I didn’t want to do. I am constantly plagued with guilt all the time. I lie awake at night often in tears thinking about how awful I am, things I’ve done. I have shared some of these things with friends who assure me they are no big deal but I can’t shake it. It makes me feel terrible. I am unable to argue with someone and then leave it… I ALWAYS have to sort things out even if it means I say I’m wrong and it’s all me. I can’t bear bad feeling. I will let people treat me however they like if it means I don’t feel guilty.

I am in my thirties now and my life looks nice on the outside. I’m so alone though. I have no relationship as i never managed to find one where the person truly loved me. I have a lovely little boy and have avoided relationships since he was 2, he’s 6 now. I earn decent money and do well at work yet I can’t fix myself. How do I stop this? I’ve had lots of therapy, nothing works, it’s like I am programmed to be open to shit treatment and guilt. I wish I could be better. How?

I don't know what the solution is for you OP but I wanted to tell you that you are not alone. I can relate to a lot of what you've said and am the same sort of age.
Apart from therapy, I've found it helpful to read self help books and articles about self compassion and to do some DBT (dialectical behaviour therapy) workbooks to develop self compassion and practice radical acceptance.
The thing that helps me the most is to try to imagine what I would say to someone I loved who was feeling like that, to try to be that person for myself. That is really hard when you feel so much self-recrimination but you have to try it if you want to change - imagine what you would want your ideal mother or older sister figure to say to you and try saying it to yourself. Guided meditation with affirmations or even hypnotherapy might really help you too.
But the main thing is, OP, that you deserve to let go of that guilt. You were not in the wrong, you are beating yourself up for no reason. You deserve to forgive and radically accept that past self who you blame so much and move forwards to become someone more empowered - but both the past you, the present you and the future you are equally deserving of compassion. You've got this 💐

I'm telling you. Start by loving yourself and being comfy with that . You deserve it. When you have come to this, you can be ok with other people

It's a mental habit to think like that. The tracks are well worn in your brain from decades of thinking the same thoughts over and over again. Any train of thought winds up gliding effortlessly onto those old tracks! You can re train your brain and make new (happy!) tracks. It just takes repetition over time.

I found Buddhism helpful at first, then Eckhart Tolle power of now, then life coaching.

You can do it and it's never too late to start.