Masturbation Easy in Sitting Position and Little Difficult in Standing?

I don't know whether you guys have faced the same issue that I am facing. Before I state my problem I would like to tell you some background about me. I have some phimosis (which means my foreskin is little tight and can show only 80% of glans when erect and I can pull my entire foreskin back when flaccid easily) and also my glans are little sensitive. My habit was to masturbate in sitting position or while lying in the bed and I looked at porn very occasionally. And finally, I am on day 5 in the reboot process.

My problem is I can get fully erect in the sitting position with some simulation and I feel difficult to get fully hard in standing position. I am very worried as I don't have any idea why this is happening.

Here are the few possible causes that I have thought of:
1) Due to phimosis, I am not feeling comfortable to masturbate in the standing position

2) It is because of my habit of masturbation due to which I have trained my brain to get erect only in sitting or sleeping position

3) I have less sensation while standing and I need a reboot to bring my sensitivity back.

4) There is some blood flow issue that is making it difficult for me to achieve an erection in standing position.

What do you think guys? Has anybody experienced the same before reboot? Will I be ok after the reboot? or is there any other cause for this?

Please let me know your valuable opinion on this as this is causing a lot of overthinking and anxiety to me.