Constipated for about 4 years

Hi. I am 20 year old male and I had constipation problems since I was about 16. I have tried going to my family doctor a couple of times, but that did not helped me so much.
Since I was 16, I had big issues with my digestive system: I go irregularly, sometimes I don't go for two days, sometimes I go once a day. It is really messed up.
I am a programmer, so I don't get a lot of exercise. However, even though I do my job sitting, I wouldn't say that I sit in front of my computer all day. Since I am a student, I need to get to my university every single day by using public transport and my feet. I do about 7000 steps I believe every single day (at least step counter app says so).
Talking about my diet: for the last couple of months I am trying to drink as much water as I can. It is not really easy since for the last couple of months I have been travelling and very busy. I eat what I like: I don't eat that much of a junk food, and since I live in one of the Baltic countries, junk food here is not that 'junky'. I do eat at irregular times: since my lectures sometimes end at 6pm, sometimes 2pm, I do eat when I can, when I have time for that. Sometimes, because of a rush I do skip a meel and in the evening eat a whole boiled horse (yes, I am that hungry sometimes)... I am trying to eat as much vegetables as I can (every meal I eat at least one tomato and cucumber).
I am 5.7" 20year old, and my weight is 136 pounds. So I am not overweight.
So, the main problem: recently my stools have been huge, so this means that going to the bathroom is painful and with blood signs. All of the time my stool is constipated at the beginning and extremely soft (sometimes too soft) at the end. Sometimes my stool is perfectly normal. Sometimes I have diarrhea. Sometimes I am extremely constipated.
All my family doctor said is that I should change my profession (which I love and won't change) and gave me laxatives. They do work, but when I stop using them, everything goes even worse.

Do you have any tips on how to get rid of this issue. I am really scared and really tired of this problem.