Sorry, this'll be long, skip it if you like of course!
I expect that I'll be most unpopular for voicing this opinion, but you asked for advice and mine is as valid as anybody else's.
First off, I agree with everyone who's said your man's sexcapade was recent and in your bed.
Second, you can kid yourself as long as you like, but he WILL continue to be a compulsive liar and he WILL continue to cheat.
Thirdly, your pregnancy isn't at the stage where the clumps of dividing cells are a viable baby, so there is zero need to feel bad if you decide on a termination.
Fourth - this man would make a shit example to a child if you had one and he stayed. (Which I'm not convinced a lying cheat would, once a baby arrived with all the concomitant sleepless nights, lack of money and no longer being the centre of your attention.)
Fifth, you are way young enough to have a baby in the future should you decide on termination - it's not your one and only chance.
And finally, which is the bit where I'm sure the Mumsnet machine guns will come out, have you really thought about what sort of future a child conceived now will have?
We have irrefutable evidence of Global Warming, and scientific predictions of the countries which will become uninhabitable due to it and the millions of refugees who will be the result of that; we have Putin, who's clearly unstable, to put it mildly, posturing about using nuclear weapons; we're heavily reliant on China, a totalitarian state with a horrendous Human Rights record, for every single category of goods I can think of, and so could be held to ransom oh so easily; our own Government is hell bent on turning the UK back to the days of the Poor House and the Filthy Rich; we have new viruses appearing and mutating whilst we can't even get on top of the 'flu ; the seas are becoming so polluted that in the foreseeable future it will become unsafe to eat seafood and to top it all off, society seems to be becoming desensitised to greed and violence so that it's no longer a shock to - for example - read about children committing murder or The Met Police humiliating and abusing a black schoolgirl on her period, or any of the other daily atrocities we learn about from the MSM.
Rant over, but I certainly wouldn't want to be responsible for bringing another life into this fucked-up world.
However, whatever you decide, I wish you luck and happiness.