What should you do if you suspect the guy you've been seeing for years is cheating?

You should confront him about it and make sure your assumption is true. If it is true then you may want to leave him since they usually say that "once a cheater is always a cheater." If you give him a second chance, he may just cheat on you again and then you end up by getting hurt even more.

AND IF HE IS tell him to suck ya mum cause she will take a young guy