If you return to the US, establish a permanent residence and get a job then you will likely be able to return to the UK for tourist purposes with a roundtrip ticket. You will get a lot of scrutiny but if you can properly demonstrate intention to return to the US then they will let you in.
Your trip to Brussels will have triggered too many red flags
- no onwards ticket out of the UK
- trip timed to occur over the expiry of your visa (so looks like a visa run)
- the employment situation
- your relationship situation
- your general lack of specific plans
In the meantime get your gf to visit you in the US and bring your stuff!