First, masterbation is totally normal and healthy! Everyone does it or has. Even animals do it! You shouldn’t make yourself feel badly about it. It’s something that humans and animals both do!
I once was listening to a sex therapist, and she said something that stuck with me. She said that when you’re alone and want to get off, any thought you want to have, have it! It’s your mind and your thoughts that make you feel good, so have them. That being said, I’m sure she meant not thoughts that are considered not “normal” like hurting other people, just sinister things in general, etc. But if for example, you were totally into sock puppets, then, have as many sexy thoughts about sock puppets that you want! Ha ha
To be serious though, it’s completely okay and normal (and healthy!) to masterbate and think of things that turn you on. It’s not hurting your boyfriend that you love, and I’m almost 100% certain that he probably has many thoughts when he masterbates. And you shouldn’t feel like you’re unclean because you masterbate. Look up the health benefits of masterbating – there’s been research done as to how it’s beneficial for health. Plus that, if you absolutely need to orgasm, it’s probably better to let yourself do it than cheating on your boyfriend (I know you didn’t suggest that at all.)
You don’t have any reason to feel like this is a bad thing:) it’s probably a good thing to do for yourself!