You've had some very good advice. I would echo what other academics have said: in the context of the range of marks that you've received, a 45% is not out of order.
Students often do less well on their undergraduate dissertations because it's the first time they've done really independent work from the start. The point of a dissertation is that you develop the topic entirely independently, and this is a big challenge - I tell my students that this is the biggest challenge of writing a dissertation - getting the topic right.
It may be that the encouragement your tutor was giving you was in the context of the level of achievement you were demonstrating. When working one to one with a student, it can be a fine balance between being very blunt about what is not working about a topic, a draft, and so on, so that the student can fix it; and being encouraging, because one makes a judgement that the student does not have the skill or talent to redraft or amend the work, the approach, the writing style (or whatever) particularly in the time given.
And it's unlikely that your marker just didn't read the last few pages. I've just marked a stack of UG dissertations, with very few annotations - we know from experience of repeated errors over 3 years in essays how little students take note of the corrections unfortunately.
Find out the marking procedures in your Department - again, the advice upthread about the process of marking is very sound. Gather your evidence, but don't go in aggressively over the head of the staff actually teaching in the dissertation module (as some pp have recommended here). It rarely does any good, as you really can't question expert academic judgement.
Academic staff are human too, you need to understand that, and there will be differences in evaluation & judgement. That's why we moderate each other's marking, so we can be sure of similar approaches and standards across a cohort. But all dissertations are different, both from each other, AND different from previous work you've submitted.