Hi Mish, perhaps you could start with what you don't want, or things that make you unhappy then approach it from the other side so to speak.
Don't feel so bad I think quite a few of us don't have any set goals for happiness, maybe more for peace and contentment. Are they so different?
Life isn't something we go along with thinking am I happy, but we usually know when we arn't.
Vision for the future is often hard to find when our immediate concerns are food on the table and paying bills. To just get by is as far as our vision goes, anything else we put in the realms of pipe dreams.
Happiness for me doesn't have to be something earth shattering or exciting, it can be found in simple pleasures like coming back from church with the Sunday papers and making some fresh coffee and a delicious danish pastry.
I am sure you have such times or similar. Don't be hard on yourself,it is not a complete lack of self respect it is a kind heart, but just be aware of him taking advantage.
I don't think it is too late to ask how long he will be staying or to say something like you don't want Marc to get to used to this arrangement so you thought he could be gone by.....

I always used to drop coins in the beggar with his dog begging bowl,knowing he went to the pub as soon as he had enough, now I put a tin of dog food down! Yep I am smart.
You can sort this, you don't have to just accept it.