I think the indecisiveness experienced by most INTPs stems from an inherent insecurity towards making a 'mistake' - either from not dialectically thinking through all the possible outcomes to their logical conclusions, or from fear of being too impulsive and emotionally-driven in our decisions. Most times, we fear that making such a mistake might eventually lead to feelings of regret - and regret is perhaps one of the things that irks the INTP the most, simply because it seems almost like a blemish to our well-guarded self-esteem, intellect, and sagacity.
One thing that has helped me overcome this dilemma (somewhat - I still retreat back into it many times) is consciously and constantly telling myself that 'regret', in its most common conception, stems purely from human vaingloriousness and egoism - and nothing more. To regret something is to knowingly ascribe to the belief that if only we had a second chance to do something, we would do it differently to fix it! Yet, actually fixing a mistake would of course mean that we'd have to be time-travelers of the mind, in true Cartesian fashion; someone who is able to transpose our future conscious selves (the selves which has become aware of the outcome of a 'mistake') into our past conscious selves (the self which was faced with a decision to make), and attaining the ability to hold the reigns to our physical selves to enact on the 'right' decision - all this to be done simply to avoid experiencing that horrendous feeling of regret!
This is of course quite ridiculous, and needless to say, impossible. And like every good INTP out there who is forced to accept a logical impossibility, the best thing to do is to not waste our time on such things. So, to answer your question, if indecisiveness is caused by a fear of regret, and regret in itself stems from an obsession with wanting to change the past, and if the ability to change the past is impossible.... then all we are left with is simply learning to go with the flow, accept the consequences, adapt, and roll with the future punches that life might bring us. This should not prevent us from engaging with each decision with an as-precise-as-possible evaluation of the situation, but it will save us a lot of superfluous deliberation since we have already recognized the absurdity of regret. It definitely helps one to come to a decision faster.
