My boyfriend is really anxious about lots of things and is constantly worried that I am unfaithful (I am not and never ever have been in any relationship).
This is such a classic trope from 'insecure' partners.
It inevitably means "I have a problem, but instead of doing anything to fix myself, I will make it YOUR problem."
It's not insecurity - it's control.
He doesn’t accuse me but will suggest that I am being sly and whilst I get that he is anxious, these digs piss me off. I have told him so, but also that I understand his anxiety and so reassure him that I am honest and not doing anything to make him doubt me.
Stop reassuring him.
Every time you do - you are 1) giving credibility to his 'insecurity' ie control 2) underlining that you're ok with him making you responsible for his 'insecurity' ie control 3) feeding his entitlement to make you jump through hoops to fix HIS problem.
I feel that the time has come for an ultimatum but not sure how to do this. Do I simply say “if you continue to accuse me/suggest I am being unfaithful then it’s over” and if he does it even once again just finish it?
A few times a WEEK?!
Yeah, you can issie an ultimatum.
But you have already told him you don't like it, & still he does it ... so what do you think would change, by saying it again?
If you give him this ultimatum, you need to be 100% cast iron sure that you can & will walk away the moment he breaks it.
Because he will.
One of 2 things will happen - he will make an effort for a short while, then his behaviour will start to re-emerge. he will apologise, promise not to do it again - until he does it again.
Or he will stop for a longer time, but his need to control you will just emerge in different ways. He'll start criticising what you wear, interrogating you about who your friends are & where you are going, he'll demand text updates as to your whereabouts, demand proof like sending him pics of where you are, & he'll start to dislike your friends & create arguments to drive a wedge between you & anyone who is not him.
It then seems so cold but I absolutely hate it when it happens. A few times a week at present although does get worse when his general anxiety levels rise.
It's not cold to protect yourself.
It's not cold to hold boundaries.
It's not cold to finish with unsuitable men.
This is your skewed sense of boundaries talking.
YOU OWE THIS MAN NOTHING.
He's already making you tense & unhappy.
What's cold about removing yourself from tension & unhappiness?
All that will happen if you stay is that his controlling behaviour will escalate.
I get that this might seem really clear to lots but for me it isn’t hence my need for help!
You've done well to recognise 1) that your boundaries are weak 2) that he is waving ALL the red flags for control & 3) that you needed a sense-check here.
What happened to you to erode your boundaries? No need to answer that if you don't want to! - but instead of wasting time on this ghastly little man (how DARE he infer you are 'sly'? Where is your anger?!), start invesitng time in yourself.
www.freedomprogramme.co.uk/online.php
www.amazon.co.uk/Woman-Your-Own-Right-Assertiveness/dp/0704334208
www.amazon.co.uk/Why-Does-He-That-Controlling/dp/0425191656/ref=tmm_pap_swatch_0?_encoding=UTF8&qid=&sr=
Get educated, get your self-esteem back up, get assertive ... & don't date anyone else until you are sure of being able to hold your own boundary - which as PP have explained, isn't about constantly informing the other person what it is, then hoping they respect it. It's about respecting yourself enough to recognise a boundary-crasher, & walk away from them.