Praise be to Allah.
Undoubtedly you have gone through a difficult period in your life, during which the Shaytaan was able to toy with you to such an extent that he made you fall into the gravest sin of all, namely disbelief in Allah after you had been a believer, then zina (fornication) and other evil deeds.
And the Shaytaan still wants to toy with you, because he is saying to you: After all these sins, you will never be fit to call people to Allah or to do such and such of other deeds of faith.
This is a trick of the accursed Shaytaan, whereby he gradually causes a person to fall into sin, then he gets power over him to prevent him from repenting. If it so happens that the person disobeys him and repents, he keeps reminding him of his past sin, not in order to make him regret it or do righteous deeds afterwards, but to keep him away from the religion of Allah and prevent them from doing good deeds, on the grounds that the one who has committed that sin does not deserve to do such righteous deeds, which are only befitting for the pure and righteous.
Once you realize this and reach this conclusion, the first thing you must do, before all else, is ensure that your repentance to Allah, may He be glorified and exalted, is sound. So first of all repent from your disbelief and previous apostasy, and understand how the Shaytaan was leading you on a path of evil which would have ended in hellfire and divine wrath, if Allah had not bestowed His mercy upon you and brought you back to His religion.
You should understand that part of complete and sincere repentance, and one of its signs, is to be keen to do acts of obedience and worship, and to keep yourself away from anything that leads to evil deeds, especially those from which you have repented and that you have given up. So establishing regular prayer, constantly reading Qur’an, carrying out the duty of enjoining what is good and forbidding what is evil, calling people to Allah, regretting what has passed – all of these are signs of having become righteous and are ways of attaining that, and they are signs of sincere repentance. So adhere to that and do not give it up, and beware lest the Shaytaan try to keep you away from any of these things because of the sin that you committed in the past or because of some false notion that he instills in your mind.
See also the answer to question no. 175916
You should strive hard in that which you are focusing on of calling people to Allah, may He be exalted, enjoining what is good and forbidding what is evil, whether that is in your home or in your place of work, or wherever you find an opportunity to do that.
Undoubtedly you yourself have seen these homeless shelters, and you have seen what is present in them of causes of fitnah (temptation) and deviation. If you can possibly find her another place to live, far away from that, then do so, then visit from time to time to call people to Allah and advise them, so long as you do not fear fitnah for yourself. But if you do fear fitnah for yourself in some place or other, then give priority to warding off fitnah from yourself over calling and exhorting others.
If you have been sincere in your repentance to Allah, then you have nothing to do with this girl apart from calling her to repent, whilst taking precautions for yourself and being careful not to go back to fitnah. If you can call her through someone else, another woman, then this is better and more prudent.
With regard to the issue of abortion, all you have to do is inform her of the Islamic ruling which forbids and prohibits it, and warn her against the sin of killing the foetus for no sin on its part, and tell her that she has no right to do that. If she responds and refrains from doing that, and you help her to repent sincerely, then that is good, in sha Allah.
If she keeps her child and does not abort it, and she does not have any money or any family who can spend on her, and you can show kindness to her and her child by spending on them and helping them as much as you can, then that would be a good thing. Allah has decreed kindness in all things, and in kindness to every living being there is reward.
But if she refuses and does not respond to you, then beware of her fitnah and beware lest she cause you to sin.
Strive hard to save yourself from the consequences of sin, by repenting from what has happened in the past and by resolving to mend your ways in the future. This should be your only focus in life today and tomorrow. Strive to protect yourself by staying away from places of fitnah and sin, and help yourself to remain chaste by getting married, in whatever way you can.
And Allah knows best.