With regard to whether inserting the finger into the rectum when fasting breaks the fast or not, there is a difference of opinion. Some of the scholars are of the view that if the fasting person inserts his finger into his rectum, he breaks his fast by doing that. 

Shaykh Zakariyya al-Ansaari (may Allah have mercy on him) said: If he inserts a finger into his rectum, he breaks his fast. The same applies if someone else does that to him with his permission. So he should be careful when cleaning himself after relieving himself, and not let the tip of his finger enter the rectum, because if any part of the finger enters it, his fast will be broken. 

End quote from Asna al-Mataalib (1/5/416) 

The second view is that the fast is not broken by anything that is inserted via the rectum, because it is not akin to food and drink. Among those who favour this view was Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him). 

Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said in ash-Sharh al-Mumti‘ (6/369): The more correct view concerning this matter is definitely that of Shaykh al-Islam Ibn Taymiyah, regardless of what some contemporary scholars say. 

One of the common forms of medical treatment nowadays is that which is placed in the rectum (suppository) when the patient is suffering a high fever; a thermometer may also be inserted into the rectum to find out the patient’s temperature; and there are other things. None of these things breaks the fast. 

Moreover we have an important basic principle for the seeker of knowledge, which is that if we are uncertain as to whether anything breaks the fast or not, the basic principle is that it does not break the fast. End quote. 

For more information, please see the answers to questions no. 22959 and 37749. 

According to this view, the one who inserts his finger into his rectum does not break his fast, according to the view that is most likely to be correct, because there is nothing in this action to indicate that the fast is broken. 

However we should point out that the basic principle is to protect the ‘awrah from such things. Whatever illness you have, there are treatments, remedies, and laxatives which you can use, instead of doing this off-putting action. We do not know of any doctor who would advise such a thing or tell a patient to do it. So you should consult some doctors who specialise in such matters, and perhaps they will tell you about something that will be beneficial in your case. 

Please see also the answer to question no. 20671 

We ask Allah to heal you from what you are suffering, for He is Munificent, the Most Generous. 

And Allah knows best.