Starting to think that there are issues beneath the surface

So, I (27, HLM) and my gf (25, LLF) have been together for 4 years.

We both work full time, mon-fri although my hours are marginally longer. I wouldn't say either of us have particularly stressful jobs.

We're both in a similar position financially, we live at my moms place currently but she's almost never there, so we have the place to ourselves aside from one night a week typically.

We've always had a very good and smooth relationship, we never fight about anything, but our sex life feels like its getting worse and worse as time goes on. In hindsight, I don't think it was ever particularly high even when we first got together compared to the previous relationship I was in and whilst I believe she has a generally low sex drive, she also suffers with IBS (frequent stomach pains) which is also a common fallback for when I try to initiate. I'd say I'm the one who initiates about 95% of the time and more often than not, I get rejected. Reasons being upset stomach, tired, not in the mood, having other things to do like going out and shopping or something that isn't remotely urgent. In the past few months, we've hit the most dry patch of our entire relationship. I'm not even intentionally counting but its easy to do when its so infrequent. We're pushing mid March, we last had sex on Valentines day, and before that on a weekend away in early December. Before that? I can't even remember, although I know it was a long time (atleast a month) as I tried to initiate before we went away and got rejected and told it'd be nice to have a weekend just us. I don't have any reason to believe that she doesn't want intimacy with me, she tells me she does, tells me she finds me attractive, and she's definitely not cheating.

We're aiming to do some travelling this year, moving to another country and working whilst there before coming home and settling down in our own place. I know our current life situation isn't perfect, but I have been trying my best to be a better boyfriend picking up most of the housework, cleaning, I do half of the cooking, I've always been in decent shape but I've been going particularly hard in the gym the past few months and even encouraged her to come with me which she sometimes does, although this has changed nothing in the bedroom.

It's only now I'm starting to get a bit concerned about our relationship and the future. We're strongly integrated into eachothers families at this point and friend groups, we've obviously spoken seriously about getting our own place and even engagement. Over the years, I've managed to sweep aside our differences in sex drive by turning to porn and masturbation, which I'm frankly tired of doing and it isn't cutting it anymore. I love her to bits and our life together but I can't think of anything worse than roping myself into serious life and financial decisions (house, ring) where I would only imagine our sex life diminishes further.

I find it really difficult to address this because she finds it incredibly awkward discussing sex generally, and in addition, I don't want her to feel uncomfortable or pressured into having sex. I've never acted out at any rejection over the years, and even though she frequently does reject when I try to initiate I can tell she feels super guilty afterwards because she looks for reassurance that we're "okay" and that nothing is wrong etc etc. I've never said there is, even if that isn't how I've truly felt deep down because I want her to feel as loved and comfortable as possible. I recognise that keeping this inside isn't healthy for me or her but for reasons stated above I haven't turned it into an issue.

I guess I'm primarily here to just get this off my chest and to see how other people have felt in this scenario but I don't really know what to do at this point. I don't want to end the relationship and certainly not without trying to make things better, but since I've finally reached a point to even post this here it's clearly playing on my mind more than I've tried to accept, and I guess my thought process is to see how the year goes with the travel plans and try to address it next time something happens and go from there