@touchoffree
First off ignore all the vitriolic nasty posts if you can. They aren't about you; they are about the poster's own life fears and problems.
I think you have a number of issues here that you have lumped together into one big one but actually they are all different.
THIS MM LOSING INTEREST Actually that is pretty normal in an affair set up. In most affairs, the MM will not leave and has no intention of leaving the marriage. He has a cosy social set up probably has children he loves and doesn't want to lose contact with and probably likes his wife even if they aren't "in love". So an affair is about sex and being made to feel good/centre of attention BUT in a situation where there is a lot to lose.
Sex/lust in early stages is a very very powerful driving force. It makes people (men and women) behave like idiots - Bill Clinton who had a hell of a lot to lose is a prime example. The power of lust means that a MM in early affair stages will be extra- keen and make his mistress feel swept off her feet.
It will wear off though because that new relationship energy (a polyamorous term but a good description of the fun of new lust) does not last. In an affair, the MM will then start to get a dose of reality and the fear of losing what he values (his family, his social circle, his wife, his financial situation) will kick in and start to override his inclination to behave like a sex ferret.
In a normal relationship, if a person is looking for a life companion and is single and there are other things to keep them interested (shared interests, shared life goals, shared point in life journey and so on) they will stay together. THAT is totally absent in an affair as there is a little prospect of it becoming the primary relationship.
So him losing interest is normal I'm afraid in an affair. It's not about you. It would happen whether you were a super-model brain box or an A-list beautiful actress.
SOLUTIONThe only solution to this is to end it yourself. You will feel better if you take control and dump him; you won't like it but you'll at least feel in control and its better than being dumped yourself especially by a MM. Tell him directly its over then delete all his contact information and block him on any social media you share with him.
YOU KEEP HAVING AFFAIRS WITH MM A one off may be put down to a genuine love/attraction/crazy lust but if it keeps happening, it is almost certainly to do with low self esteem in two ways.
You fear no one really wants you and you will always get rejected, so an easy way to deal with that is to hook up with a man who is not available. It means you can mentally comfort yourself with the fact that it isn't really rejection when he ends it, because he was never yours to start with.
You also feel that you don't deserve happiness so you subconciously set yourself up in a situation where you know you will not get happiness but will get pain.
SOLUTION Big challenge here - but you need to re-build your self esteem which is obviously rock bottom. Start by avoiding relationships and dumping this man. Then take a good hard look at your life - what don't you like about yourself? Then work on changing whatever it is. Feel unattractive - get down the gym, get fit. Feel you have no friends - start doing regular activities where you meet the same people regularly - a course, volunteering in the same place, a club. Spend time only with people who genuinely like you, avoid people who put you down and make you feel like shit.
ALL RELATIONSHIPS ENDING WHEN MAN LOSES INTEREST This is a different problem again because a single man is in a very different position to a MM.
I think again this is likely to be related to your self esteem. Low self esteem leads to bad decisions - picking bad men because that's all you feel you deserve; staying in a bad relationship trying to make it work when you can see they aren't careing for you and you should walk away.
SOLUTION Take a break from dating, work on your self esteem and then when you feel better approach dating like you would finding a job that is your dream job. You'd think about exactly what you'd want that would make you happy in a job and then go and look for THAT job or one like it. Focussing on your values and interests is a better way to go than looks for example. Eharmony as a dating site has this type of approach but there are others.
Good luck and I hope you feel better soon.