Hi!
I just wanted to share my background with you, as the DH and I dated for 5 years and saw each other frequently. I *always* told them I was going to see my boyfriend/fiance/husband; I always had proof of my job and whatever apartment I was renting, and I never, ever had any trouble. Not once. Occasionally, it took about a minute to get through the line, but rarely more than 20 seconds maybe? They would ask when I was leaving (and I was usually staying for at least three weeks, and the entire summer vacation sometimes), and I would occasionally be asked how I was supporting myself (usually prompting a laugh by replying "on, my enormous salary as a teacher") and that...would be it. The only time I did have trouble, I was WITH DH as we'd just come back from a weekend break in Paris, and the IO wanted to know if I was re-entering to restart my visa. Since that hadn't even occured to me (and I was leaving 2 weeks later anyway!), I said "uh, no" and after a few more questions, she let me through.
I understand that you are worried about this, and it is super stressful, but the reality is that IO's are not there to stop you from visiting your boyfriend, they are there to prevent you from breaking the law, if you're not doing that (and can demonstrate it), it shouldn't be too terrible.