I'm sorry that you're having a hard time with your daughter's choices. I think it's a good thing that your daughter broke up for feeling smothered, and knows her priorities. This guy does sound very needy and may have addictions issues, and i don't think that is attractive to her. I think it's a good choice that your girl made, and i think you need to support that choice. One way of doing that is to not get involved in her personal life in unless she asks you to. She's learning to be independent and she sounds like she's got a good head on her shoulders. She doesn't want a man that calls her mother when she doesn't text back, who would? Her choice is a good one and very understandable. It would help if you saw all this from her perspective.
That being said, it sounds like you are a bleeding heart, and I think you would truly be a blessing to join an organization that helps homeless singles and families.(attending homeless shelters and helping with the food and lodging needs). I'm sure there's plenty of opportunity to talk to these folks that need your support, maybe play a game of cards, help someone with a clothing need, or maybe be a driver to help collect used goods, There's so much to do for those in need. The love you feel in your heart for this young man, could be put to good use by being present in people's lives that need to feel that kind of love. Not that this boy doesn't need that, he just doesn't need to stay stuck with your daughter who is not interested in him (and shouldn't feel like she would make anyone happy if she were, other than herself). She HAS to think about putting herself first and finding her way knowing she has your full support.
As for the ex boyfriend, i think you should tell him that he needs to talk to a therapist and that it's not healthy for you to act as a go between for him and your daughter. I think that by you staying involved with this young man, you are essentially helping him to stay stuck to a situation that he needs to move on from. I don't think that you will be able to help him effectively, but i think that you can direct him to a therapist that can. So you can still effectively help him by separating from him and letting others take up the slack.
You know, as a parent of a teenager, I do find it kind of sort of odd that you as the mother really want your 17 year old to have a boyfriend this bad. AND one who was a heavy pot smoker with some issues. That he is hanging on like that and 'showing up' is a bit stalker ish and concerning. Immaturity can cause this as can emotional problems but as her mom, make sure to NOT encourage him in ANY way.
Is there a dad in the picture? The reason I ask is that you seem to be longing for a man to be around. He fits the bill with being male. because otherwise, this doesn't sound like a good situation for anyone other than he is a male that is in your house from time to time. Try not to project onto your daughter if this is the case. Encourage her to focus on school and her girlfriends. My hope is that my sons don't get involved with all of the dating distractions until they are done with college. lol. I know, good luck with that. But I definitely know they would be better off if that were the case.
good luck
Yes, I am sure you will. No matter how hooked you got on this young couple and their issues, these are their lives, and their lookout, and they will find their own feet. It is not you, it's not your relationship it is not your life. You focus on you.
You will help yourself a lot if you find something more fun and fulfilling to do. Try volunteer work in an area you really care about, or asking for some more interesting but random responsibility at work (not necessarily something you get paid for, just something new that you like). Go to some new places. Try some new foods. Take a weekend in a different locale. Walk on the beach. Climb in the mountains.
Your daughter is still going to go through a lot of changes as she grows, and so is her ex boyfriend, they are both terrifically young. That will happen as part of their natural development. It's totally not surprising that they are in and then out of each other's lives. They sound like they are not surprised by the shift away from each other either, and have adjusted. You need to do the same. If you can't, see a counselor for help.