To tell his gf ?

[Kimberley00001]

Been sleeping with this guy on and off for a year and a bit , yep you guessed it he has a gf ( or maybe a few he gets around ) I feel very angry about how he has treated me and I'm wondering do I tell his gf? She deserves to know what he is , and I suspect she left her husband to start relationship with aforementioned guy. Wibu to tell her?

If you want to tell her, do it.
But at least be honest with yourself- you aren't doing it out of the kindness of your heart, you want to hurt him the way you feel he's hurt you by dumping you. She got what you wanted, and now you want to knock her down a peg or two.

Do it if you must, but don't dress it up as having honourable intentions.
Don't expect her to believe you, either.

Your story just keeps changing.

It's basically consistent in that you have been a long standing booty call for him, but the little details keep changing.

Stop lying to yourself, if you can't be honest with yourself then you've no hope quite frankly.

You are jealous that his latest gf has captured his attention so much that he has cut you off.
Up until now, you felt like you were 'winning' because he would keep coming to you throughout all of his previous relationships.
This time it's different - he likes her too much (at this time) to risk losing her over a casual fuck.

You are mid 30s you say?
Then stop wasting your best years on a twat and move on.
Don't continue the drama by contacting his gf just because you're pissed off with him.

It annoys me tbf he's done this behaviour for years and he walks away with out a scratch...

and why shouldn't he? Unless he made promises of exclusivity to you that he didn't keep then he owes you nothing. People are entitled to flit from relationship to relationship if they want to, they are entitled to be players if they want to. So long as they are not lying and deceiving you when they are sleeping with you then you really don't have any grounds for complaint.

If they are lying and deceiving someone else while they are sleeping with you, well you can hardly get all morally upright and indignant about that after you've been dumped, if you were happy to play your part in that deceit all the while he wanted to keep shagging you.

It always smacks of spite when people say 'she deserves to know what he's like.'

You don't give a shit about what she deserves.