What can I do to have my destination confirmed? I have seen a lot of great buddhists who change their paths due to some kind of conditions. Recently, I also face a that kind of situation. When, I faced a dissapointment, I forgot all teachings that I have learnd. It got some time to understand the situation. It just a small one and I had knowledge to understand the situation. There may be some situations that out of my understandings.

What can I do to have my destination enlightenment , confirmed ?

Related question: What makes a monk to be a house holder ?
Im not sure about guarantees but there are these relevant passages;


  Association with people of integrity is a factor for stream-entry.
  Listening to the true Dhamma is a factor for stream-entry.
  Appropriate attention is a factor for stream-entry.
  Practice in accordance with the Dhamma is a factor for stream-entry.
  — SN 55.5
  
  There is the case where a monk remains focused on unattractiveness with regard to the body, percipient of loathsomeness with regard to food, percipient of non-delight with regard to the entire world, (and) focused on inconstancy with regard to all fabrications. The perception of death is well established within him. [One such as this] attains the immediacy that leads to the ending of the effluents. -an4.163 (excerpts)
  
  Admirable friendship, admirable companionship, admirable camaraderie is actually the whole of the holy life. When a monk has admirable people as friends, companions, &amp; comrades, he can be expected to develop &amp; pursue the noble eightfold path. -sn45.2
  
  When, on observing that the monk is purified with regard to qualities based on greed, qualities based on aversion, qualities based on delusion, thinking: 'There in this venerable no such qualities based on greed, anger or delusion, with his mind overcome by which qualities, he might say, "I know," while not knowing, or say, "I see," while not seeing; or that he might urge another to act in a way that was for his/her long-term harm &amp; pain he places conviction in him. 
  
  With the arising of conviction, he visits him &amp; grows close to him. Growing close to him, he lends ear. Lending ear, he hears the Dhamma. Hearing the Dhamma, he remembers it. Remembering it, he penetrates the meaning of those dhammas. Penetrating the meaning, he comes to an agreement through pondering those dhammas. There being an agreement through pondering those dhammas, desire arises. With the arising of desire, he becomes willing. Willing, he contemplates (lit: "weighs," "compares"). Contemplating, he makes an exertion. Exerting himself, he both realizes the ultimate meaning of the truth with his body and sees by penetrating it with discernment. -mn95 (xcerpts)


As to related question. The analogical reasoning in the sutta is smth like this 'there are dangers on the journey, as there are for a sailors; sharks, waves, weather and whirlpools . That some warriors tremble and run away hearing drums of the enemy, some tremble at the sight of their banners, some are defeated in hand to hand combat, some get injured and recover, some get injured and do not recover.'

I assume many things may cause one to disrobe, longing for this or that, perhaps one has to take care of parents. 

If one disrobes it doesn't necessarily mean that one's practice stagnates. I think pali lore has a story of a monk disrobing, becoming enlightened and rejoining the order.