Bismihi ta'ala
At first, we must understand the practice of istikharah, when/how/where and why it is done. If we do not correctly understand the purpose and usage of istikharah, we will face problems.
Marriage itself and pursuing a case for marriage is not something that should be dependent on istikharah. If you take the correct avenues, with having family and elders involved, and investigating and asking about the life and behaviour of the prospect spouse, then you are going to reach a conclusion.
If you have prioritised the correct requirements for a spouse, in being religious, and in having high akhlaq standards, then the investigation and consulting with others will give you your answer. There would be no need to resort to istikharah.
Istikharah would only be applicable if after everything you are still two-minded, and you need to seek divine intervention.
In any case, it is not haram to go against an istikharah. Of course, it's certainly better to not disregard the answer given to you by istikharah, or why else did you turn to do it in the first place.
If you did an istikharah, and it turned out against what you deeply wanted, and you wish to pursue it, then pay some sadaqah, allow some time to pass, change the circumstances, and then decide whether you wish to pursue it, or do another istikharah.
Always bear in mind that just because istikharah came out good, does not mean your marriage will last. You still need to be all your effort in keeping a marriage, and maybe it is God's fate for you to go through whatever is destined for you. That's why the best thing is always to make sure your spouse selection is based on the solid criteria recommended by Islam.
With prayers for your success.
