If you are in a relationship and the person changes their personality, could this be a sign of cheating?

I'm in a long term relationship, were the person is in the military and constantly separated. Once in the relationship, he was very affectionate. Now he acts like he never wants me around him for long, and constant disappoints me and seems like his cheating, or doing some else wrong behind my back. His personality has changed, and only seems to care about things at work, and seems like he constantly lies. But when I get feed up and have had enough, tell I'm going to leave, he begs me to stay and saids I'm the only one who cares about him, and I'm the only one he really loves. But then it seems like he is doing everything wrong to get me to leave? He then sometimes says he doesn't want to be togther, but then always takes it back and we end up staying togther. He is driving to the up the wall. Do you think I should leave or do something to jolt him? Please help! I love him very much still but need to have a piece of mind for once. I just want some certainty in my life. I don't want to leave just because of suspecions. Please help. I have talked to him till I'm blue in the face, nothing seems to work. I saying goodbye the only answer? Please help. I usually have sound judgement and he is confusing me.'