Praise be to Allah.
It seems that in your case you are suffering from waswaas (whispers from the Shaytaan) with regard to matters of purification. This is a serious matter, because waswaas may make some people resent worship and find it burdensome, so it feels as though it too difficult. What you must do is take measures to rid yourself of this waswaas. The best remedy for waswaas is to ignore it and not pay attention to it; you should not take any notice of what the Shaytaan is trying to stir up in your mind of doubt concerning purification, prayer and the like. At the same time you should also offer a lot of supplication (du‘aa’), calling upon Allah, may He be exalted, and asking Him to grant you wellbeing and to help you to overcome the Shaytaan. See the words of al-‘Allaamah Ibn Hajar al-Haytami (may Allah have mercy on him) on dealing with waswaas, for he offers useful advice, which we have quoted previously in fatwa no. 62839. For information on dealing with waswaas having to do with istinja’, please see fatwa no. 100268
With regard to how to do istinja’ and clean oneself with water after urinating, that is done by pouring water on the place where the urine comes out, then washing it gently, whilst taking measures to lighten the pressure of the water coming out of the tap or hose, so that the pressure does not lead to the water splashing onto your body. Then you should wash off the impurity, as all people do.
If you cannot reduce the pressure of the water on the place where the impurity is, then take some water in a small vessel or even in your hand, and wash your private part with it when you have urinated, three times only. By doing that you will purify yourself of urine.
You should do the same thing when defecating; try to reduce the pressure of the water so that drops of impurity will not splash onto your body.
If you cannot do that, then take some water in your hand and wash the place where the impurity is.
You can also use tissue first, and remove the impurity by wiping three times with the tissue, then if you wish you can use water after that. You can do that and it will not matter if any water splashes onto you after that, if you cannot reduce the pressure of the water, because the place has been purified by wiping it with the tissue first.
And Allah knows best.