I'm nearly 19 and I've been with my boyfriend for nearly 2 years. When we first got together we had a pretty good sex life - 3-4 times a week and sometimes we'd do it a few times in the same day. After about 6 months we went through a rough patch and he ended up cheating on me. I didn't find out till a year later. However I did notice a change in our relationship, especially sexually. We'd have sex once every couple of weeks and one time I found out he was watching porn instead of coming to pick me up. I have mentioned how I feel on a couple of occasions and it's a different excuse each time; he's tired, works stressful, he's not feeling well, he's got something to do in the morning etc. When I found out he had cheated on me I was hear broken and I explained how I felt and told him I would only stay if the relationship got better. He has told me that this was the first and only time he has cheated. I work in a clothes shop at the moment and sometimes I work quite late so I don't see him or speak to him for most of the week. So that has left me to the thought that he is cheating again or he has replaced me with porn. I hate talking about it with him because I feel like I am nagging him and I know it doesn't make a difference. I just don't know what to do anymore.

You talk to him, that's what you do. You are in a relationship. This is just one of the problems you will probably have.

I'm not suggesting you accuse him or scream at him - ask him why you don't/can't speak with him all week. Maybe it's scheduling, maybe it's something else. He presumably is the only person who knows for sure.

#1... got over it with the porn excuse. ALL normal guys like porn and watch it to varying degrees. THose of us that are straight still like women... the gay guys still like guys... and the bi guys still like both... and the ones into bestiality... are just as sick as ever.

First off... the relationship likely isn't all that anyway... not in his eyes. People drift apart for as many reasons as there are people. And it has NOTHING to do with cheating.

And the biggest lesson in life you could learn right now... nagging a guy is the best and most efficient way to get rid of him once and for all.

Talking is fine... talking is good... nagging on the other hand isn't talking... its dictating.. and nobody man or woman wants talked down to.

There's too many ifs here. If he's cheating and feels guilty or satisfied after having cheated... if he's having a mentally sexual issue such as only being able to get off in certain situations or in certain ways that do not include you.. if he's just not wanting the relationship anymore and can't break it to you.
The main thing here is your young and this isn't worth your time. If he was trying to figure it out with you then Id say he's worth the pain.. but he's not... a relationship is a partnership.. it doesn't sound like you have a partner to help you with this issue. Its time to find someone who is going to be your partner in crime. People are people- they're real and you can relate to them and their emotions you can connect and fall in love with anyone. Having feelings for him doesn't mean he's right for you. But it does prove you can love someone new since you fell for this guy and he's obviously not it.